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When seeing a hot girl or thinking about one gives you an erection. Used jokingly to exaggerate the size of one's penis.
"Great, thinking about you in a swimsuit just made me break my zipper."

Guy 1: "Dude, did you see that chick?"
Guy 2: "Yeah, dude! I just broke a zipper!"
by M4d Baumer June 15, 2010