Stupid or stubborn; lacking intelligence or refusing to change one's mind despite obvious reasons to do so.
From 'bonehead' (a stupid person) + '-ed' adjective suffix. 'Bonehead' emerged in American slang around the 1890s-1900s, with the exact origin debated but possibly referencing a thick skull.
The term 'bonehead' became famous in baseball—it's still used to describe a dumb play—and became so popular that by the 1920s, it was standard American slang, showing how sports terminology shapes everyday language!
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