A variation on the classic motorboat technique (see below) normally applied to breasts, the Larsen Outboard is instead applied to butt cheeks.
Motorboat: The act of pushing one's face in between two ample breasts, and rocking one's head side to side very rapidly while making a vigorous, lip-vibrating "brrr" sound.
Moosy (Larsen) asked Steady to cook him a cheeseburger.
Steady replied, "Sure, right after you finish motorboating my butt cheeks."
A loyal friend and hungry for a cheeseburger, Moosy obliged him with a Larsen Outboard.