"to mend (fabric) by interweaving yarn or thread to fill a rent or hole," c. 1600, of unknown origin. Perhaps from French darner "mend," from darne "a piece, a slice," from Breton darn "piece, fragment, part." Alternative etymology is from obsolete dern "secret, hidden." Related: Darned; darning.
tame curse word, 1781, American English euphemism, a minced form of damn said to have originated in New England when swearing was a punishable offense; if so, its spread probably was influenced by 'tarnal, short for Eternal, as in By the Eternal (God), favorite exclamation of Andrew Jackson, among others (see tarnation). Mark Twain (who spells it dern) writes “this imprecation is a favorite one out in the ranching districts, and is generally used in the society of ladies, where a mild firm of expressionomy may be indulged in” (San Francisco, 1865). Related: darned (as a past-participle adjective, 1806); darndest (superlative, 1844), darnation (noun of action, 1798).