Someone who sits in a public toilet taking a shit as silently as possible, not moving around or making any noise, with the intent of catching someone doing something embarrassing thinking they are not there.
It is usually a good idea to duck down and check for shit ninjas before you do something that would otherwise be considered embarrassing.
Joe: *walks into bathroom, thinking nobody is in there*
Joe: FUCK, that slutty little bitch was SO motherfucking hot! But too bad she's only twelve years old! *slams wall with fist*
*sound of rustling toilet paper*
Joe thinks: (Oh god, it's a shit ninja)
Mike: *walks into bathroom, thinking nobody is in there*
Mike: I gotta take a massive fucking dump! *FAAAAAART*
*sound of someone shifting around on a toilet seat cover*
Mike thinks: (fuck, not another shit ninja)