This is when you place you face straight into a females cleavage/breast and do a lip trill making the sound of a motor boat and shaking your head from side to side.
It usually tickles.
"Motor Boat me baby!"
Charlie smiled as he quickly shoved his face into the big breast and made a motor boat sound shaking his head.
The motor boat is done on a girl's clitoris,(putting your lips together and blowing air through them) making a motor boat soundlike a raspberry or zerbert. a lot more fun than in between the tits for both people.