The enlarged head of a bolt that is struck with a hammer; also, a person considered foolish or reckless.
From 'bolt' plus 'head,' referring to the mechanical part or used as an insult similar to 'blockhead.'
Calling someone a 'bolthead' was a period-specific insult from the 20th century—it's largely disappeared, replaced by 'knucklehead' or 'bonehead,' showing how insults evolve.
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