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.

A dunce. A dummy. A very stubborn person or animal.

a person who doesn't listen, and stays in trouble

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".

A nickname associated with fans of the heavy metal band Five Finger Death Punch.

someone who is irresponsible and acts ignorant

the punching of a boner in attempts to make it go away.