It's slang for making a buzzing noise while moving your head side to side between a woman's breasts.
Here's what Urban Dictionary has to say about it...
v. int. The act of pushing one's face in between two ample breasts, and rocking one's head side to side very rapidly while making a vigorous, lip-vibrating "brrr" sound.
n. The sound produced when doing a motorboat.
"Wouldn't you love to see Meatloaf motorboat Barbara Bush?