A mild oath or exclamation expressing frustration, annoyance, or anger; a euphemism for 'damnation'.
An Americanism and euphemism derived from 'damnation' (from Latin 'damnare' meaning to condemn). The alteration to 'darn' and 'darnation' allowed speakers to express strong emotion without the religious offense of 'damn'.
This is a perfect example of how people create euphemisms—'darnation' lets you express genuine anger while staying socially acceptable, which is why it became the go-to frontier-era exclamation.
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