Origin: prob. < 'tarnal (short dial. form of eternal) + (dam)nation
See tarnation in American Heritage Dictionary 4
Origin: + (damn)ation. Regional Note: The noun and interjection tarnation illustrate suffixation, the addition of a suffix to a word. Tarnation and darnation (the latter probably having come first) are both euphemistic forms of damnation. Tarnation seems to have been influenced by tarnal, another mild oath derived from (e)ternal! The Oxford English Dictionary cites late-18th-century examples of tarnation from New England, indicating that it has been part of American speech since colonial days.
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