To have sex, usually very quietly and in a fashion that will avoid detection. From the phrase "Ninjathat bitch."
Most often heard in the gerund form "ninjing".
ninj (v.): to act in such a manner that could be construed as causing trickery, disruption, and/or pain on those who instill one's wrath
Ninj (n.): any one of a number of people who enjoy, and in fact take delight in, causing mischief and trickery on those who deserve it; i.e., mauling someone
v. "I'mgonna ninj you, you bloody prick, if you ever say something like that again!"