They could be a figurative assninja, or a real gay ass ninja!
The latter would dress up as a ninja, with stars and everything, and perform ass sex with kunais, advanced forms of taijutsu, ninjutsu and genjutsu.
They are to be feared when they possess a sharingan!
Yeah, one of the rapists was Gay-Sensei
Also, Ass Ninjas are a threat used by parents, telling their children that if they don't go to sleep Ass Ninjas will get them.
Go to bed now or the Ass Ninjas will get you!