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1.
The art of masturbating in a high/semi-high traffic area in your home or in a public area without anyone catching you or being suspicious of anything.
All of a sudden I got really aroused in a public place so I had to ninja fap
by FUCKSHITBITCH13245215125346486 April 06, 2011
 
2.
Its when you secretly beat off for someone else to find later.
"Every morning I ninja fap in the shower. And my dad thinks thats soap between his toes."
by dave_dew January 19, 2007