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One who is stealthy like a ninja, jumping from one ass to another (usually raping them, usually a gay ninja too).

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!
John walked oppenlegged, ass bleeding out of his bedroom this morning. He had a swarm of assninja's on his back all night, and won't be able to sit for a month now!

Yeah, one of the rapists was Gay-Sensei
by Fukaface! March 26, 2019
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One who engages in anal sex so quickly and stealthily that the other person is unaware that it has occured.
Bobby was in and out in 9 seconds. He is a true ass ninja.
by FAH Q 2 January 18, 2006
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Stealthy like a Ninja but Clumsy like an Ass
" I snuck up behind this dude while playing Halo and then like a total AssNinja, I got excited and threw a grenade and ended up killing myself instead" :(
by King Spaz January 18, 2011
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ninja's that spontaniously spawn from ones ass.
'Goddam, i had a bit of trouble with some ass ninja's this morning.'
by Prycey September 09, 2003
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A mythical being who exists to give people wedgies. As with the Tooth Fairy, Santa Claus and the Easter Bunny, the Ass Ninja visits all the little boys and girls in order to bunch their underwear at the worst possible times.
"Dude I totally just got visited by the Ass Ninja! Cover me while I pick my underwear out of my butt."
by Dark_Angel August 25, 2012
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One who runs away after licking ass.
"Your mom is such an assninja. She ran away before I could pay her for one of her award-winning rimjobs."
by Faust December 25, 2003
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A ninja warrior who, quite literally, kicks ass. They aim straight for the buttocks and pummel mercilessly. See also: 'munkfuck
"Man, we got so raped by those ass-ninjas!"
by Snnnnickerdoodle January 24, 2007
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