Stupid, foolish, or lacking intelligence; characteristic of a dumb head or foolish person.
Adjectival form of 'dumbhead,' combining the noun with '-ed' to describe someone possessing the quality of being a dumbhead.
By adding '-ed' to a noun we already made an insult, we doubled down—'dumbhead' was bad enough, but calling someone 'dumbheaded' made it an inherent personal quality rather than just a name.
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