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"
By corgilover18
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.
By BMan01
A great way to end an awkward conversation with a girl you don't want to talk to.
"This girl was in my room and I couldn't take it anymore so I motorboated her. After the motorboating she left."
By A Bobby
The act of making a monotone hum while pinching and moving the cheeks back and forth
We thought Michael was getting lucky in his room, but it turned out he was just motorboating by himself.
By BobbyHi11
The act of going down onto a female and making a buzzing sound with you lips.
Bro, last night i moaterboated that girl! HAHA you motorboating son of a bitch!
By DGAFinc.