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 bigbreast 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.