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.
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!"