A stupid or foolish person; someone who lacks common sense or intelligence.
From 'numb' (unable to feel) + 'skull' (head). The idea is of a head/brain that's 'numb' or unable to function properly, originating in 17th-century English.
Numbskull is a brilliant insult because it literally imagines a person's brain as 'numb'—unable to process information—which is why it sounds worse than just saying 'stupid'; it suggests the person can't even feel their own thoughts.
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