sternwheeler

v. int. The act of pushing one's face in between the buttcheeks, and rocking one's head side to side very rapidly while making a vigorous, lip-vibrating "brrr" sound. Like a motorboat, except with the butt rather than the breasts.

Oh my God, Becky, look at her butt. It is SO BIG. I'd love to give that thing a sternwheeler.