The act of someone acting like a complete retard only to destroy his own reputation in attempt to gaining one
kid: Hello folks, i am your nerf ninja

others: wow, what a faggot
by IamPerson1 December 29, 2010
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The act of nerfing something without any forewarning.

While most nerfs are announced that they will happen before they actually happen, a ninja nerf is simply done, leaving the players to discover, usually being completely screwed over at the same time due to being surprised.
Player A: C'mon, let's go kill that boss. I'll get my shifter tank.

Player B: Ok, let's go. Those shifter tanks get like 2000 hp, so we should be fine.

*Player A and B get to the Boss*

Player A: omfg Wtf!? I've only got 800 hp!? *dies*

Player B: Oh Fuck... *dies*

Player A: Well wtf... I had 2000 hp yesterday... I didn't see anything about a nerf on the forums, did you?

Player B: Nope. Must have been a ninja nerf.
by H-o-G August 11, 2006
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