The stealth and silent act of masturbating with other people in the room. A properly executed Ninja Defuse leaves all other people in the room unknowing of your deed. Most common in college dormitories and sexless relationships.
Guy 1: Dude i'm really horny, i need to go rub one out but my roommate is always sitting around playing Call of Duty.
Guy 2: Bro you need to try a Ninja Defuse.
Guy 1: WTF is that?
Guy 2: Just throw your covers over yourself in bed, and masturbate while he isn't paying attention, but keep quiet or you'll get caught!
Guy 1: Oh damn son! You're a genius!
Of the game Call of Duty 4:
When playing the gametype "search and destroy", a Ninja Defuse consists of sneaking up behind the member of the attacking team who is planting the bomb at either target A or B and lying in wait behind them for them to finish planting. As soon as they have done so, one begins immediately to defuse the bomb (occasionally actually pulling it from underneath them). The enemy who has just planted the bomb rarely check behind them for a ninja, so concerned are they with watching nearby corners for oncoming enemies. Most effective when all but one member from each team has been eliminated.
Game announcer man: Bomb defused.
Bomb planter:Aw man i just planted! How the hell he defuse that?
Ninja:Aha n00b i just ninja defused you.
Term "ninja defuse" comes from Counter-Strike. It's when Counter-Terrorist player stays hidden near C4 and waits for all remaining Terrorists to exit area before C4 explodes. Then, with defusal kit, he can quickly defuse C4 and win the round.
Most notable example is from SK v NiP match from CPL Winter 2005.
Commentator 1: Oh, he's allowing them not to know that he's there. As it is - He's gonna go do - We're gonna be looking at ninja defuse.
Commentator 2: OMG! OMG!
Commentator 1: He's gonna be able to sneak right in there. Work his way in there. Here comes the defuse!