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.
-"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."
I totally ripped her a new asshole, but it was her idea to knuck fuck.