1. Slang term used for any male douchebag who is rude, disrespectful, belligerent, violent, angst-filled or otherwise aggravating on account of the fact that he can't get laid.
2. Any young male whose only real use is pumping protein samples into socks or garbage pails.
3. A griefer in an MMO.
1. "Hey knuckle fucker, sit down and shut the hell up, and stop making a scene because you're a limp-dicked little douchebag that couldn't get his dick wet if he jumped in a fucking lake."
2. Tuef, Bum, Toneslice, Noobslice, etc.
3. "I'm trying to get my gear fixed but there's some knuckle fucker spawn camping the repair guy." Also see #2.
This is someone that is too stupid to be a Jackoff.
Biff is such a Knuckle Fucker, he can't get anything right!
A man who masterbates
John knuclefucked himself before he went to sleep.
One who graps his penis with his hands in a white knuckle grip and vigorously jacks his cock off.
That David is such a knuckle fucker.
A Lesbian. A homosexual female.
No chance mate - shes a knuckle fucker