A clumsy, stupid, or slow-witted person; someone who is awkward and lacking intelligence.
From Old Norse 'álfr' meaning elf, which became associated with deformed or changeling children left by elves in medieval folklore. Over time, the meaning shifted from 'offspring of elves' to 'stupid clumsy person' as elf-lore became less believed.
Medieval people actually blamed their oafish children on elves—if a kid seemed slow or clumsy, they thought fairies had switched their real child for an elf-baby, which made 'oaf' basically medieval slang for 'probably a changeling.'
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