(Friend A) "did you have fun with John Smithlast night?"
(Friend B) "It was alright, but he Ninja-fucked me and im not into that. It was so quiet you could hear crickets!"
When you ( a male) is having sex with two other people ( male or female) and you fuck them in the butts back to back. Hence, you see an ASS, you see an ASS, and you went IN, or assassin ( which is a word used to describe a ninja).
Joe Schmoe: Hey bro, I just had a consecutive buttsex with these two smoking hot chicks last night
Douchebag Dan: Duuuuuude! You just had a Ninja Fuck!!!
Joe: Totally gnarly dude!!
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.
Guy who likes to screw chicks violently and fast, like a ninja, sometimes in a public place. The girl is usually someone the "ninja fucker" has just met.