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 heardJimmy 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-cut top and Mike approached her and started motorboating in her cleavage.
A pornographic act describing getting a blowjob from a girl with pigtails - holding the pigtails in each hand and pulling her head in so that she gargles on your penis, making a sound like a motorbike.
Dude, that chick will go down on anyone, she is so hardcore she is in to motorbiking!