On the ninja seat
When you're in the bathroom and trying to take a quiet crap if someone is outside the door. Or when ther is multiple people in the room at once
Tim was on the ninja seat when Judy was waiting outside the door for him.
Driver Seat Ninja
When the driver of a car does a karate chop to scare the passenger.
Fred was scared by the driver seat ninja, the random and abrupt Karate chop causes Fred to poop a little.
car-seat ninja
Children that are under 50 lbs and taking any type of martial arts class.
Damn, do you see all of those car-seat ninjas outside of the dojo yesterday evening?, Cosmo Kramer pwnd a bunch of car-seat ninjas.