when a guy pisses on the rim bowl of the toilet (the white space just above the water) so that it won't make a splashing noise while your peeing.
(a guy thing... i hope)
"It was like 3 in the morning and i didn't wanna wake up my parents so i just took a quick ninja piss"
When one vertically climbs up a piss stall using his hands and legs with his junk hanging out, and pee pee as best as he can towards the direction of the yellow bowl. He would then release the grease with a very Asian karate chop sound, thus completing the Ninja Piss.
"dude, i heard the bathroom stalls are fucked"
"oh hell yeah nigga, Jimmy totally ninja pissed that shit like a pro asian G"