Frog in a Bag
The act of pelvic thrusting in your underwear to make your male genitalia flop from your belly button to your gooch area. The illusion is similar to the sight of a frog jumping around in a bag.
Tim: Hey Paul, come here you gotta see this.
Paul: Better not be a Frog in a Bag, I don't wanna see your sack flopping around anymore.
Paul: Better not be a Frog in a Bag, I don't wanna see your sack flopping around anymore.