a ninja rock is a rock that sneaks up on you while walking or running and causes you to either twist, sprain, or break your ankle. At the very least it will make you say ouch that hurt. Most commonly happens at night when you cant really see whats on the ground so well.
I was walking to the chow hall with my roomies and out of nowhere this ninja rock made me twist
Aww fuck damn ninja rock just tried to take me out. I hate those sneaky
Dude Caleb was walking
to back from the store last night and stepped on a ninja rock. It put him on crutches