If I heard this right, it refers to the action of being sexed half-willing, whether out of obligation or whatevermotivation (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.