Nipple ninja 

A person who can grope a females breasts without being caught or causing conflict

That guy is a fucking Nipple ninja

Nipple ninja by Nothunterself December 16, 2013

Nipple Ninja 

A member of a not-so-secret and vaguely homoerotic society originating in Danville, California. A Nipple Ninja is fluent in many techniques of nipple flipping, and performs his art on unsuspecting victims. These moves (examples include: The horseshoe, the rainbow, the monkey wrench, and the Horny Hungarian) can be performed in friendly or unfriendly situations.

Holy shit, that Nipple Ninja just pulled a "guillotine" on Madlwski's fat nips.

Nipple Ninja by Steve Kaye December 4, 2005

Nipple Ninja 

What a baby is called when displaying innate ability to fend off removal from the breast when feeding.

"Agghhh, the Nipple Ninja is chewing my nip, but I can't get her off!!!"

Nipple Ninja by Annie's Mum May 11, 2009

Stealthy Nipple-Ninja 

When you're about to cum, twist the girl's nipples as hard as you can so she freaks out and screams and is distracted, oblivious to the fact that you're blowing inside her

The day before my two weeks notice was up was the same day my asshole boss gets back from Sweden; therefore, I pulled the stealthy nipple-ninja on his wife. That'll learn em'

Stealthy Nipple-Ninja by Brandon/Steve April 8, 2013

Ninja Nipples 

What a woman has if she goes all day without a bra on and no one noticed.

We were out all day with Jane and never noticed she wasn't wearing a bra. It was nipply out and she wasn't wearing a jacket either. The woman's got ninja nipples!

Ninja Nipples by ShrewdSimian May 31, 2010