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The action of fucking one with ones knuckles, most likely in the asshole, however, this can cause ripping in the anal walls, so lube up and avoid practice if knuckles are extremely bony, or you have a constant twitching problem, and or phobia of blood.
We were so drunk last night, she let me knuck fuck her.

I totally ripped her a new asshole, but it was her idea to knuck fuck.
by james o'harris September 09, 2010
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Someone who doesn't get any pussy.
-Always fucking their own knuckles: masterbating.

To finger blast a chick.
-Penetrating the vaginal cavity with one's knuckles.
Use in colloquial dialogue:
-"Atleast I'm not a knuck fuck like Andrew!"
-"Dude, all I had to do was knuck fuck a bitch and she got all wet."
by Vin November 12, 2007
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