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In reference to the Ninja/Ninja Pro perk of the CoD games. It's basically a fancy way of saying that you don't have trouble being quiet/sneaky.

Most often used when your friend is worried about you making noise when sneaking out of or into your house.
Teen 1: C'mon man, let's go!

Teen 2: Did your dad hear you?

Teen 1: Relax buddy, I'm running Ninja right now.
by AL_PS3 February 10, 2011
In reference to Call of Duty games, it is a fancy way of saying "Nobody will hear me" when you are sneaking into or out of a house or other building you weren't supposed to leave/enter to begin with.
Buddy: Dude, are you seriously going to come with me later? Won't your parents hear you?

You: Relax, I'm running Ninja up in this biatch.
by AL_PS3 February 14, 2011