Stupid, thick-skulled, or lacking in intelligence; the adjectival form of 'beetlehead.'
From 'beetlehead' (noun) plus '-ed' suffix creating an adjective, similar to 'thick-headed' or 'mule-headed'—a pattern for insulting adjectives describing personality flaws.
This insult combines entomology with psychology—the idea that hard, closed beetle shells couldn't let ideas in made 'beetleheaded' perfectly capture stubborn stupidity in a way 'dumb' simply cannot.
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