We all know that there's no actual need to know Latin, but OTOH, there's always a gobsheen who trots out some rote-learned phrases in order to impress -- so here's a few phrases as a poke in the eye to that sort of behaviour.

They begin with 'everyday' Latin phrases, ones that you come across from time-to-time in english text, or even conversation -- so that we can hold our own against the amadáns; and then I've included a few slightly ... less common ... phrases at the end, for those special occasions, when you really want to take the piss.
Enjoy.

ab initio
from the beginning

absit omen
may the presentiment not become real or take place

ab urbe condita
from the founding of the city.

ad hominem
(arguments) directed against a person rather than against his arguments.

ad litem
appointed for a lawsuit

ad rem
to the point; without digression:

Ad valorem
According to its value

advocatus diaboli
devil's advocate

Ad infinitum.
To infinity.

Ad nauseam.
To the point of sickness.

Ad litteram.
To the letter.

Ars artis gratia
Art for the sake of art

Ars longa, vita brevis.
Art is long, life is short.

Agenda
Things to be done

Alea iacta est.
The die has been cast.

Ad astera per aspera.
To the stars through difficulties.

amor patriae
love of one's country; patriotism

anno regni
in the year of the reign.

annus mirabilis
a year of wonders, catastrophes, or other notable events

Anguis in herba
Snake in the grass

argumentum ad hominem
argument to the man, rather than to the facts.

Ave et vale.
Goodbye and farewell.

Actus reus
the act, which in combination with a mens rea constitutes a crime

Bona fide.
In good faith.

carpe diem
lit. sieze the day
enjoy the pleasures of the moment, without concern for the future.

casus belli
1. an event or act used to justify a war.
2. the immediate cause of a quarrel.
[literally: occasion of war]

Caveat Emptor
Let the buyer beware

cogito ergo sum
I think therefore I am.

compos mentis
of sound mind

Condemnant qui non intelligent.
They condemn who do not understand.

Consilio et armis
By plans and by weapons

coram populo
in the presence of the people, publically.

corpus vile
a person or thing fit only to be the object of an experiment.

cui bono
for whose benefit? for what purpose?.

Cum grano salis.
With a grain of salt.

Cum propris suis alis volat.
He flies with his own wings.

decus et tutamen
an ornament and a safeguard

de dicto
Lit. about the saying
Logic, philosophy. relating to the expression of a belief, possibility, etc., rather than to the individuals mentioned, as in the number of the planets is the number of satellites of the sun,the truth of which is independent of what number that is. C ompare de re.

de fide
from faith
R.C. Church. (of a doctrine) belonging to the essentials of the faith, esp. by virtue of a papal ruling.

Dei gratia
by the grace of God

demisit sine prole
died w/o issue

de novo
from the beginning, new.

Deo gratias
thanks be to God, DG.

Deo volente
god willing. dv

de profundis
out of the depths of misery or dejection.

de re
Lit. About the thing.
adj. Logic, philosophy. (of a belief, possibility, etc.) relating to the individual rather than to an expression, as the necessity of the number of wonders of the world is prime since that number, seven, is necessarily prime.

Deus
God

deus ex machina
Lit. God out of a machine.
(in ancient Greek and Roman drama) a god introduced into a play to resolve the plot.

Dies Irae
Day of Wrath.

disjecta membra
scattered fragments, esp. parts taken from a writing or writings.

Dominus
God, or Christ.

Dum volvo, video disco.
While I turn, I see and learn.

Dum spiro, spero.
While I breathe, I hope.

Dulce et decorum est pro patria mori.
It is sweet and fitting to die for one's country.

Ecce Homo
Behold the man.

Esse quam videri.
To be rather than to seem.

Ex animo
From the heart.

editio princeps
the first printed edition of a work.

e pluribus unum
one out of many: the motto of the U.S.A.

exempli gratia
eg
for the sake of example

exeunt
they go out: used as a stage direction.

exeunt omnes
they all go out: used as a stage direction.

Ex nihilo nihil fit.
Nothing comes from nothing.

ex silentio
lit. from silence
(of a theory, assumption, etc.) based on a lack of evidence to the contrary.

ex voto
in accordance with a vow.

Et alia
And others

Ex officio
Out of office

Ergo
Therefore

facile princeps
an obvious leader.

fecit
(he or she) made it: used formerly on works of art next to the artist's name.
Abbrev.:fec.

Festine lente
Make haste, slowly.

Fidei Defensor
FD
Defender of the Faith

floruit
(he or she) flourished:
used to indicate the period when a historical figure, whose birth and death dates are unknown, was most active.
Abbrevs.:fl., flor.

fons et origo
source & origin

genius loci
1. the guardian spirit of a place.
2. the special atmosphere of a particular place. [genius of the place]

Gloria in Excelsis Deo
glory to God in the highest

Gloria Patri
glory to the Father.

hic jacet
(on gravestones) here lines

honoris causa
for the sake of honour

Habeas corpus.
You have a body.

I.E. Id Est
That is

idem
the same
used to refer to an article, chapter, etc., previously cited.

id est
ie

ignotum per ignotius
the unknown by means of the more unknown]

in absentia
in the absence of

in aeternum
forever; eternally.

In camera
In chambers (ie in private)

In Deo speramus.
In God we trust.

Index Librorum Prohibitorum
"the index"
RC church's list of forbidden books

in extenso
at full length

in extremis
1. in extremity; in dire straits.
2. at the point of death.
[literally: in the furthest reaches]

infra
(in annotation) below, further on.

In hoc signo vinces.
In this sign, you will conquer.

in loco parentis
in place of a parent:

in medias res
into the midst of things

in perpetuum
for ever

in propria persona
in person

in rerum natura
in the nature of things

in situ
in the natural, original, or appropriate position

inter alia
among other things.

inter alios
among other people

inter se
among or between themselves.

inter vivos
Law: between living people

in toto
totally

in utero
within the womb

in vacuo
in isolation; without reference to facts or evidence.

in vino veritas
ìn wine truth

In flagrante delicto
Caught in the act

ipso facto
By the fact itself

ipse dixit
an arbitrary and unsupported assertion.

ipsissima verba
verbatim

Lapsus linguae.
Slip of the tongue.

laud aute dominum
praise the lord

literae humaniores
Lit. the more humane letters
the faculty concerned with Greek and Latin literature, ancient history, and philosophy; classics.

loco citato
in the place or passage quoted.
Abbrevs.:loc. cit., lc.

loquitur
he (or she) speaks: used, esp. formerly, as a stage direction.
Usually abbreviated to loq.

Lux sit
Let there be light

maxima positio
greatest proposition

Media via
In the middle way

Magnum opus.
Greatest work.

mare nostrum
Mediterranean

mea culpa
my fault

Mens Rea
Lit. Guilty mind.
In legal circles, it's used to mean 'criminal intent'

mensa
the table

mens sana in corpore sano
a sound mind in a sound body

meum et tuum
mine and thine: used to express rights to property

mirabile dictu
wonderful to relate; amazing to say.

Miserere mei, Deus
Have mercy on me, O God

modus ponens
mood that affirms
In logic, the principle that whenever a conditional statement and its antecedent are given to be true its consequent may be validly inferred, as in if it's Tuesday this must be Belgium

modus tollens
mood that denies
Logic. the principle that whenever a conditional statement and the negation of its consequent are given to be true, the negation of its antecedent may be validly inferred, as in if it's Tuesday this must be Belgiumand this isn't Belgium so it's not Tuesd ay.

mutatis mutandis
the necessary changes having been made.

Multum in parvo
Much in little

Morituri te salutamus.
We who are about to die salute you.

N.B. Nota Bene
Note well.

ne plus ultra
the extreme or perfect point or state.[literally: not more beyond (that is, go no further),
allegedly a warning to sailors inscribed on the Pillars of Hercules at Gibraltar]

nihil
nothing, nil

Nil Desperandum
Never despair

non licet.
it is not permitted

non liquet.
it is not clear

Nolle prosequi
We shall no longer prosecute

nolens volens
whether willing or unwilling

non compos mentis
not in control of one's own mind.

Nosce te ipsum.
Know thyself.

Nolite Timere
Be not afraid

Nemo Me Impune Lacessit
No one touches me with impunity.

nulli secundus
second to none

Nolo contendere
I am unwilling to contest.

Non sequitur
It does not follow

omnia vincit amor
love conquers all

Omnia amor vincet.
Love conquers all.

onus probandi
burden of proof

opus anglicanum
English work

ora et labora
pray and work.

ora pro nobis
pray for us

O tempora! O mores!
oh the times! oh the customs!: an exclamation at the evil of them.

passim
here and there;

pater familias
is head of family, or head of tribe

paternoster
our father

pax vobiscum
peace be with you

per ardua ad astra
through difficulties to the stars: the motto of the RAF.

Perlculum in mora.
There is danger in delay.

per mensem
every month

persona grata
a welcome person

persona non grata
1. an unacceptable or unwelcome person

Per se
By itself

pinxit
he (or she) painted it: an inscription sometimes found on paintings following the artist's name.

Prima facie
At first view

primum mobile
a prime mover.

primus inter pares
first among equals.

pro bono publico
for the public good

Pro Tem (Pro Tempera)
For the time being.

pro patria
for one's country

pro tempore
pro tem
for the time being

Possunt qui posse videntur
They are able because they seem to be able.

quantum meruit
as much as he has earned: denoting a payment for goods or services in partial fulfilment of a contract or for those supplied when no price has been agreed

quod erat demonstrandum
which was to be proved.

QED Quod Est Demonstradum
That which has been proven.

Qui docet, discit.
He who teaches, learns.

Quid custodiet ipsos custodes?
Who shall watch the watchers?

res
a thing, matter, object

rus in urbe
the country in the town

RIP Requiescat In Pace
May he rest in peace

salva veritate
without affecting truth-value.

semper fidelis
always faithful

semper paratus
always prepared

senatus consultum
with the advice of the senate

Sic
Thus

sic passim
thus everywhere
a phrase used in printed works to indicate that a word, spelling, etc., occurs in the same form throughout.

Sic semper tyrannis.
Thus always to tyrants.

sic transit gloria mundi
thus passes the glory of the world

sine die
w/o a day fixed.

sine prole
w/o issue
demisit sine prole
(died without issue)

sine qua non
without which not
an essential condition or requirement.

sub judice
under judicial consideration

Sub rosa.
Under the rose (secretly)

summum bonum
the principle of goodness in which all moral values are included

sursum corda
up hearts
lift up your hearts

Tabula rasa
Blank slate

Tempus fugit
Time flies

Terra incognita
Unknown land

ultra vires
beyond authority

ubi supra
where indicated / cited above.

urbi et orbi
to the city and the world

ut infra
as below

ut supra
as above

via media
the middle way
a compromise

vi et armis
by force and arms

Veritas numquam perit.
Truth never dies.

Viz. (viclicet)
Namely

Veni Sancte Spiritus
Come Holy Spirit

Viva voce
By the living voice

Viam inveniam aut faciam.
I will find a way or I wil make one

Veni, vidi, vici.
I came, I saw, I conquered.

Die dulci fruere.
Have a nice day.

Raptus regaliter
Royally screwed.

Si hoc signum legere potes, operis boni in rebus Latinus alacribus et fructuosis potiri potes!
If you can read this sign, you can get a good job in the fast paced, high-paying world of Latin.

Ne auderis delere orbem rigidum meum!
Don't you dare erase my hard disk.

Catapultam habeo. Nisi pecuniam omnem mihi dabis, ad caput tuum saxum immane mittam.
I've a catapult. Give me all the money or I will fling an enormous rock at your head.

Sentio aliquos togatos contra me conspirare.
I think some people in togas are plotting against me.

Noli me vocare, ego te vocabo.
Don't call me, I'll call you.

Cave ne ante ullas catapultas ambules.
If I were you , I wouldn't walk in front of any catapults.

Nullo metro compositum est.
It doesn't rhyme.
Non curo. Si metrum non habet, non est poema.
I don't care. If it doesn't rhyme, it isn't a poem.

Utinam barbari spatium proprium tuum invadant
May barbarians invade you're personal space

Utinam logica falsa tuam philosophiam totam suffodiant
May faulty logic undermine your entire philosophy.

Mellita, domi adsum.
Honey, I'm home.

Te precor dulcissim supplex
Pretty please with a cherry on top

Magister Mundi sum
I am the Master of the Universe

Estne volumen in toga, an solumtibi libet me videre?
Is that a scroll in your toga, or are you just happy to see me.

Prehende yxorem meam, sis.
Take my wife please.

Recedite, plebes. Gero rem impreialem!
Stand aside plebians! I am on imperial business.

Vescere bracis meis.
Eat my shorts.

Fac ut vivas.
Get a life.

Stultus est sicut stultus facit.
Stupid is as stupid does.

Cogito, ergo sum.
I think, therefore I am.

Cogito, ergo doleo.
I think, therefore I am depressed.

Carpe diem.
Seize the day.

Vah! Denuone Latine loquebar? Me ineptum. Interdum modo elebitur.
Oh! Was I speaking Latin again? Silly me. Sometimes it just slips out.

Non plaudite. Modo pecuniam jacite.
Don't applaud. Just throw money.

Peruniunt montes, et mus ridiculus nascitur.
The mountains are in labour, and a ridiculous mouse is born.

Parve leves capiunt animos.
Small things effect light minds.

Credo quia absurdum est.
I believe it because it is unreasonable.

Noli habere bovis, vir.
Don't have a cow, man!

Vis eccum erit ... semper.
The Force will be with you... always.

Sit vis nobiscum.
May the Force be with you.

Sic biscuitus disintegratum
That's the way the cookie crumbles.

Quidquid latine dictum sit altum viditur
Anything said in Latin sounds profound.

Veni, vidi, visa.
I came, I saw, I did a little shopping.

Vestrum excrucibo.
Your torturer.