Green’s Dictionary of Slang

motorboat n.2

(US) the act, typically performed when a stripper is giving a lap-dance, of pushing one's face into her cleavage and rocking the head from side-to-side.

[US]S.M. Jones Lives Laid Away [ebook] ‘Speaking of tits, that chick who gave you the lap dance and motorboat jug job? You get off on that or what?’.