Vulgar french word meaning to fuck (as in sexual intercourse).
Can also mean to kiss, in which case it is usually used as a noun (ex: faire un baiser à quelqu'un).
Je vais baiser ta mère la pute! (I'm going to fuck your whore of a mother).
Baise-moi, saloppe! (Fuck with me, bitch)
Le soldat fît un dernier baiser à sa femme avant d'aller se faire exploser la geule à la guerre. (The soldier gave a last kiss to his wife before going to get his head blown off at war