In long-ago England, it was believed that elves sometimes secretly exchanged their babies for human babies—a belief that served as an explanation when parents found themselves with a baby that failed to meet expectations or desires: these parents believed that their real baby had been stolen by elves and that a changeling had been left in its place. The label for such a child was auf, or alfe, (meaning “an elf’s or a goblin’s child”), which was later altered to form our present-day oaf.
Ref: Merriam Webster
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