A sexual act that is done to someone while they are asleep, passed out from being drunk, or drugged up by giving them a roofie in their drink.
Bill saw Sharron passed out for being too drunk so he gave her the Bill Cosby Sleeper
Slipping sleeping pills into an alcoholic beverage and give that to their date and have sexual intercourse with them without them knowing it.
Bill was mad at his girlfriend hasn't had sex with him in a long time so he gave her a Bill Cosby Sleeper.