enculer, va te faire
The rudest thing you can say in french without using your imagination. Translated, it means "go fuck yourself", and is the closest that French has to offer to the great British "fuck".
Simply by learning a few more basic words - including dog, mother, suck, tetraplegic, vodka and full anal - you can do much better.
written by excluded pupil, Jo* Bly*h, approved by Log

Mr Carnell taught us that the rudest thing you can say in French is "Et ta soeur!"
Translation: "And your sister!" It apparently worked best as a reply to someone insulting you; i.e. "You're a tosser!" "So's your sister!"
I suspect that this is the rudest thing you can say in french. IF YOU'RE FIVE YEARS OLD.

written by An*y Ma*sh, approved by Log

Here is a quick lesson in the many different ways you can fuck the sister of a friend, in the style of those hilarious emails which give show you the flexibility of the word "fuck". You know... "what do you mean, a fucking iceberg?" Those. They're funny.
J'encule ta soeur.
"I fuck your sister", or "I am fucking your sister". It should be obvious from the circumstances which one you mean.
J'ai enculé ta soeur.
"I fucked your sister." Bog-standard.
Je vais enculer ta soeur.
"I am going to fuck your sister." Best said whilst putting your hat on and leaving the room with a monkey wrench in your hand.
Je dois enculer ta soeur.
"I must fuck your sister." Say this after banging your head in a "that reminds me, I'm so forgetful" sort of way.
J'enculais ta soeur, mais elle a mal au vagin.
I would fuck your sister, but she has a poorly vagina.
You could always do the pluperfect, I suppose, but I really can't be bothered. And according to Altavista, enculer means to fuck in the ass. So that's a nice bonus for everyone concerned.
written by Jo* Bly*h, approved by Log

More literally, enculez means bugger rather than fuck. French slang for bottom is 'cul' (think of culottes or cul-de-sac) so enculer is to 'enbum'.
'Vous vous enculez' is apparently vastly more insulting to a French than 'go and bum yourself' sounds to an English.
Why am I writing this? I found all this out in my thirties, not in the playground. Soz.
(That's alright, Doctor Jaysus; your insight into rudimentary French swearing has been valuable, and had "bum" in it. - Log)
written by Do*to* Jayu*, approved by Phil