This is a word to describe how girls are fake, dishonest and evil to each other by doing sly, under the radar things to one another i.e. passing mean unsigned notes, sending rude texts, tagging things on bathroom walls, spreading rumors, leaving people out..you know bitchy like a mean girl, quiet and sneaky like a ninja.
As opposed to being a cool chick.
Related to the noun, ninja-bitch
"Uhhgg she is such a ninja-bitch, I know she's the one doing it."
"Stop being soo ninja-bitchy, just say what you need to say to my face."
"We should totally 'accidentally' go spill our drink on her...Well maybe not, may be that's a little too ninja-bitchy"