Other forms: knuckleheads
A knucklehead is a thick, blundering fool. You knucklehead, a bunch of knuckleheads, some knucklehead did it.
It is American, informal, and usually affectionate, which is what separates it from a real insult: a knucklehead is somebody being stupid rather than somebody who is stupid. It was popularised by wartime training films.
1nounknuckleheadA mild American slang word for a thick, blundering fool, often said with affection.
“Hey, knucklehead, you've put your shirt on inside out!”
knucklehead is a fool.
Also accepted in games: nitwit, lummox
From knuckle + head.
Etymology from Wiktionary, CC BY-SA