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A person who is very subtle or sneaky when it comes to looking at a female's breasts.

Used as a compliment by males and a warning or an insult by females.
Girl1: Bob said that he didn't masturbate. He's so lying.

Girl2: I dunno. Maybe he's telling the truth. I mean, I've never seen him look at a girl's boobs before.

Girl1: That's because he's probably a boob ninja.
by Littleman29 February 24, 2010
A ninja that is addicted to boobies and possibly cocaine.
Woah, that boob ninja was totally just checking me out.
by The Waffler December 08, 2005