look up any word, like bangarang:
To have sex in an extremly fast, violent, but pleasurable (for both parties) event. The idea of "Fucking like a Ninja" is to get in, out, and off, before you can get caught, be it by parents, angry spouses, or even the police, in some rare occasions.

She got ninja fucked, dude. Right after they both got off, he was gone like a goddamn shadow.
by Andrew Flanagan February 26, 2006