To penetrate another person without that person having knowledge or noticing.
Person 1: Whoa what was that?
Person 2: You just got ninja fucked!
To have sex in a quick and intense manner.
"I could tell by the look on her face that she got ninja-fucked last night."
To have sex in a bunk bed without your bunkmate noticing.
Dude, I ninjafucked this girl last night. My roomie had no idea. I guess he likes being rocked to sleep.
If I heard this right, it refers to the action of being sexed half-willing, whether out of obligation or whatever motivation (this isn't referring to rape, mind you). It comes from a movie called Dog Park, and I'd like to see the term used more.
"How'd it go last night? Well, she ninjafucked me."
wearing a black-body suit with only your dick n balls hanging out in a very dark room, drunk ho walks in u tap that shit with your baby dick, and peace out with her having no idea...ninjafucked.
God damn im so fucked up in ninjafucked.