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1.
A sexual act performed by a man for his partner. The man will shit on the face of his partner then wipe it off of the eyes in a horizontal direction with his penis. The partner will be left with a shit-mask resembling that of a ninja. Then, the man will insert his shit-covered cock into his partner's mouth when they are least expecting it, thus leading the partner into a ninja-like attack stance by the sudden insertion.
Steve begged me for a Dirty Ninja last night but I decided to use a nicely aged Alaskan pipeline on him instead.
by DirtyNinja April 14, 2008
 
2.
A variation of the Dirty Sanchez but for real ninjas. When conducting a Dirty Ninja the Ninja in question will take his penis out of his womans ass and wipe fecal matter all over her face leaving only the eyes visible. After you have accomplished this you must proudly proclaim out loud "Dirty Ninja" to be an offical Dirty Ninja.
ex. After having sex with my wife too soon after a delicious crave case, she was punished with a Dirty Ninja

WARNING: A Dirty Ninja is not to be taken lightly, loose peanuts or corn may cause skin lacerations or eye injuries
by Tbagr June 20, 2010