A nasty ass, musty smell that permeates the apartment of a horny couple or a high-sex dorm room. The smell usually builds over time because the couple fails to practice proper hygiene, clean the sheets or open the windows. Usually causes normal people to want to vomit.
Couple A: Come on in, make yourselves at home. Sorry the apartment's a bit messy.
Couple B (to themselves): Damn, their apartment has that sex smell. Disgusting. (They vomit.)
Dorm Guy 1: Dude, open some windows, this place is developing a nasty sex smell. I can't take it anymore.
Dorm Guy 2 and Girlfriend: Really? We don't smell anything.
Dorm Guy 1: That's because you're used to it, filthy animals. Now clean your fucking sheets and let's open up some windows.