darn (v.)
"to mend" c. 1600, perhaps from Middle French darner "mend," from darne "piece," from Breton darn "piece, fragment, part." Alternative etymology is from obsolete dern (see dern). Related: Darned; darning.
darn (interj.)
tame curse word, 1781, American English euphemism for damn, said to have originated in New England when swearing was a punishable offense; if so, its spread was probably influenced by 'tarnal, short for Eternal, as in By the Eternal (God), favorite exclamation of Andrew Jackson, among others (see tarnation). Related: darned (past participle adjective, 1806); darndest (superlative, 1844).