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A chronic, habitual masturbator. Someone who is always giving himself a tug. Typically addicted to porn, stalks Facebook, Insta and other media for pics to fap off to.

May also be used as a derogatory term for somone who’s a POS.
My neighbor is a real jack knuckles. He’s always giving himself a wank online.

My uncle is a jack knuckles. I catch him tossing off every time I walk past the den.

Fuck that guy. He’s a jack knuckles.
via giphy
by Eaton Holgoode December 08, 2017
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