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1.
A bowel movement that is stealthy, silent, and quick, but results in total destruction. The defecation community's equivalent of a Silent But Deadly fart.
Ron: "Dude, I Just took a Ninja Dump! Stay out of the bathroom!"
James: "But you were only gone for a minute, and I didn't even know you were in the bathroom!"
Ron: "Precisely!"
by Pat Hamm April 17, 2006
 
2.
When you take a dump on the side of the toilet bowl instead of in the water, with the purpose of making as little noise as possible to not let anyone know you're taking a dump.
The walls are so thin in my apartment, I always have to take ninja dumps when we have parties here.
by mornfang July 15, 2011
 
3.
To take more than half a roll of T.P. and cram it with a plunger and then take a crap in the bowl, and giving it a single flush to lodge the T.P. and poo, in the Toilet, then when finished wipe your ass with the T.P the entire roll and leave it on the floor and to take the Plunger with you so they can't unclog the toilet. they will flush and flush but to no progress. simply leave with their plunger knowing your king of the lowest move at a party ever and on the housekeepers most wanted list.
hey lets go no one's gonna be able to be near that room i just took a Ninja Dump in there here quick hide the plunger.
by LargeLONEWOLF22 August 01, 2014