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1.
From the 1930's (circa). Refers to a person of questionable intelligence. The size of the brain being given relative size of a human knuckle. Similar to pinhead.
That guy just sat on a bench with wet paint. What a knucklehead.
by The Phat Doctor December 27, 2004
 
2.
Someone of limited intelligence and with a mean nature. Who prefers using their knuckles/fists, to using their head.
That doorman looks a real knuckle head
by IanM September 14, 2003
 
3.
A dunce. A dummy. A very stubborn person or animal.
I told him how to do it, but he would not listen, the knucklehead!
by Merrily McCarthy July 17, 2003
 
4.
a person who doesn't listen, and stays in trouble
Dat boy a knucklehead he stay in trouble.
by J-Wilk May 06, 2007
 
5.
Engine produced by the Harley Davidson Motorcycle Company from(1936 to 1947). The engine consists of an overhead-valve v-twin design with covers on the valves that resembled knuckles, hence the name of "knucklehead".
Mike's knucklehead got repo'd after he missed his second payment
by The B-rad October 13, 2006
 
6.
Someone who has just made a mistake or done something wrong. Often used in a joking manner between friends.
After arriving at the ski resort Pat discovered that he had forgot his skis to which his friends responded "Pat, you're a knuckle head!".
by zantern June 22, 2005
 
7.
the punching of a boner in attempts to make it go away.
dude I got a boner in the parking lot so I gave myself some knucklehead to make it go away.
by knuckleheadwittness January 26, 2012