Silly, foolish, or stupid people; numbskulls or blockheads.
A 20th-century American slang term combining 'knuckle' (referring to a blow on the head or simple-mindedness) with 'head.' It likely emerged from the idea that someone is thick-headed or has knuckles for brains—literally not smart enough.
This word perfectly captures American humor: instead of just saying 'fool,' we say 'knucklehead' to make it sound affectionate. It's the kind of word you'd use for friends doing something stupid—mean but funny, which is very human.
Complete word intelligence in one call. Free tier — 50 lookups/day.