ninja wank 

verb: to masterbate in the presence of others without their knowledge, typically in a bedroom situation. A skill used by frustrated individuals who must share sleeping arrangments with another who wants nothing to do with them sexually but must still fulfill their sexual needs.

She said she had a headache, so I waited until she went to sleep and ninja wanked.

Dude, sharing a room with my brother sucks. He's always ninja wanking when he thinks I'm asleep. It's freakin' nasty!

ninja wank by tinygoat June 6, 2005

Ninja Wank 

the art of wanking quietly and discretely in public or in an area of high visablity, or simply the art of wanking like a ninja.
A secretive faction of monks from long and far have mastered this form of masturbation whilst going out there daily rituals.
It is said to bring inner peace and balance to ones self, but costs a fortune in laundry bills.

only the masterful eye can see someone having a ninja wank, but if someone does this would be the reaction...

man 1- 'Dude wtf is that man doing in his pocket?'

man 2- 'Must be having a ninja wank'

man 1- 'Holy shit i thought that was just in fairytales'

Ninja Wank by Shaggy Quinton June 24, 2010

Wank Ninja 

Someone who is very skilled at wanking covertly in the same room as another person - the master of risky wanking. The Wank Ninja can successfully wank in a number of locations without being detected: in the back of the car with their family, under the table during a meeting, on the sofa next to a friend, etc. The art of the silent finish is vital, as is suppressing any hint of the Orgasm Face.

The other night, my girlfriend totally managed to get herself off three times during dinner with my grandparents - she's such a wank ninja.

Wank Ninja by TheWN June 8, 2012