[JSeery]

1784, American English alteration of darnation (itself a euphemism for damnation), influenced by tarnal (1790), a mild profanity, clipped from phrase by the Eternal.

The longer version: The history of the noun and interjection tarnation illustrates a phenomenon called taboo deformation, by which words that can be considered offensive or impolite under some circumstances are made more acceptable or softer-sounding by altering one or more sounds that make up the words. Tarnation appears to be a variant of darnation, itself a taboo deformation of damnation, in the same way that darn is an alteration of damn. Tarnation, however, seems to have been influenced by tarnal!, another mild oath that is derived from the adjective eternal—as used in eternal damnation! for instance. The taboo deformations darn, darnation, and tarnation have been used in New England since the late 1700s.