A sex position in which a man positions his body on a couch in a perpendicular fashion. With his head on the floor, and his legs lying on the the upright part of the couch. The women then subsequently sits on top of his crotch area, so as to have one leg on the side of the man, and the other hanging off the back of the couch (or standing). Thus she fits on the man like the top of pen!
My girlfriend and I did the pen top last night. It was awesome