The communal act of rough...wild...passionate...primatial fornication. Usually accompanied with various vocal tones and frantic leg hmuping usually seen and heard from orangutans. It is also customary to wear "PLanet of the Apes" costumes in order to successfully portray monkey sex.
Dude, I was SO stoned, that I actually agreed to have monkey sex with this hot chick. *shameless expression* It was nice. ;)