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".

someone who is irresponsible and acts ignorant

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

An honors physics student of ability but who lacks the conventional expression of said ability (i.e. homework completion, class appropriateness). May or may not get high grades.