v. int. Similar to motorboard; however, the act of pushing one's face in between two barely-there breasts, and rocking one's head side to side very rapidly while making a vigorous, lip-vibrating "brrr" sound.
You can motorboat Deb because her breasts are triple D's. You can only motorboard Linda because she's so flat chested.
When you try to motorboat a girl who is flat chested.
Deb is too flat chested to motorboat, I'll just have to motorboard.