(nin-juh up) verb. The act of tying a black t-shirt to ones face and doint subversive activities. Usually used when stealing cigarettes or blowing up peoples cars.
Johnny Twobags - Man, Ame has a pack of Newports, but shes asleep and I'd have to crawl over her.
Mr. Dopey - You'd better ninja up and do that shit
*Johnny Twobags ties his shirt up and backflips off the ceiling.
It is said of the situation where a person has the bad luck to make contact with his testicles against an undefined surface or object, intentioned or not.
Given the nature of the word, it is more appropriate to design cases where the interaction is made with a moving object, for example, a ball.
Although it is extremely painful for the victim, it tends to be considerably funny to people who witness it.
Today in the baseball game the pitcher took a nutshot; the baseball hit him in the nuts.
Man, I just watched the funniest nutshot video ever.