The placement of one's face, specifically the mouth, into the area between a well-endowed woman's breasts, followed by a rapid shaking of the face in a side-to-side motion accompanied by yelling. The resulting sound that is created sounds similar to an outboard boat motor.
Sherry was wearing a low-cut top and Mike approached her and started motorboating in her cleavage.
I like slapping the boys around the face with my boobs. I have yet to experience a motorboat however!