How to say it: Pronounced "be-zay."
What it means: In the old days, it simply meant "to kiss." Currently the official equivalent of "to fuck." A good reflexive verb: Ils se baisent—"They're fucking."
When it's used: When you ain't talkin' about love.
Raunchy relatives: Baisodrome—word coined by French high-schoolers to describe a particularly orgiastic make-out party. One of the all-time great naughty neologisms.

David McAninch is assistant editor at a New York publishing house (not dirty books) and used to teach French curse words to his students when he was a French T.A. in grad school and had run out of material half way through the class.