1.(adj) Used to describe something that is so awesome it blows your mind. Comes from the fact that ninjas are well..ninja.
2.Can also be used as a characteristic to describe a person or thing.
1. "Did you see those gangstas on scooters rockin there kickass skillz before??" "Yeah, it was so ninja!"
2. "That girl is so ninja for starting the use of the word ninja in many people's daily life.
3. Daybies think that's so endo when really it's not even ninja.