A sex prank. Its when the guy is going doggy style on a girl. When everything is going good...The guy pulls out quickly and sticks it in the girls butt. The object is to then stay on as long as you can while she tries to get away. (Sometimes the girls agonizing moan from the pain of getting it in the butt sounds like a bulls groan)
Dude...the other night i totally pulled "The Bull" on this one girl!!!