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1.
A poop ninja is one that silently poops their pants in the presence of others.

An alternate version according to www.poopninjas.com is a pooper scooper trained in the arts of the ninja.
Ex. 1 A poop ninja needs to go to the bathroom but thinks they can hold it. They accidently let some slip out in their pants, but keep a straight face. Their mood and mannerisms do not change, and no one can tell any difference.

Ex 2. A poop ninja stealthily cleans a yard full of dog poop.
by pooppooper March 10, 2011
 
2.
When one takes an accidental shit in their pants, and then blaming it on a fictional character, sometimes refered to as just "someone", or the "Poop Ninja"

Can be used as a verb, as well as a noun, sometimes even both.
Example 1: Someone took a shit in my pants, I don't know who, and I don't know why, but this is an issue that needs to be addressed immediately

Example 2:
Jimmy to Ricky: Dammit! Look out man, The Poop Ninja is on the loose. He got me.
by Big_Mac October 26, 2010