It means the activity of putting one's face between a woman's breasts, and rocking turning one's head rapidly from side to side while making a noise like a motorboat.
"Put your head between her tits and go bbl bbl bbl bbl bbl..."
"I heard Jimmy was motorboating Bella at school
she said it tickled"
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-cuttop and Mike approached her and started motorboating in her cleavage.
n; One who motorboats --
n; Term applied to one who puts there head in between a females breast, moves there head side to side makeing a motor boat sound.