One who sneaks about at a party, slipping rohypnol (roofies) into peoples' drinks to try and see who will succumb to its effects and then make an attempt to get lucky with that unlucky person. Usually a roof fairy is a loser who can't get laid without drugging someone.
Bill- Hey Joe, what's the matter? you look like shit!
Joe- Man, I got visited by the roof-fairy at the club last night. I wanna kill whoever did that!