The act of making an engine noise when sticking your face in a woman's BOOBS.
Michael: Did you hear what John did last night?
Buck: Yeah, I heard he was motor-boating that hot piece of ass.
Michael: Yeah, sick she was UGLY.
Buck: But it wasn't down there, it was up top.
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.
"Motor-boating her tits and go bblbblbbl bbl bbl..."
Like motor boating but to a nut sack. When a man puts their lips and places then in between two testicles and blows with tight lips soaked in saliva while shaking their head back and forth in a quick-like motion. The effect is the sound somewhat similar to a motor boat.
Mike: check out that nut-meal.
Aaron: motor broating that all morning long and probably late into the night sounds like fun. If my lips are a boat then I have a full tank of gas.