Used in reference to
sitting on something in a sexual manner. The phrase first came into being during an episode of "The Girls Next Door" when
Hef's new twin girlfriends were doing a sexy tennis photo-shoot. Both were holding tennis racquets and the photographer asked them to do something different. One person who was watching the show shouted, "
Sit on it!" in reference to the girls sitting on the tennis racquets in a sexual manner.
Photographer in show: Ok girls, now do something different with the
tennis racquets.
Guy 1 (watching show): Ya, ya, sit on it!
Guy 2 & 3 (sitting next to Guy 1), stare at Guy 1 in utter amazement, disgust, and
intrigue.