Origin of tarnationprobably ; from 'tarnal (short dialect, dialectal form of eternal) + (dam)nation
New England & Southern US
Origin of tarnationAlteration (influenced by tarnal) of darnation. Word History: 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.
(countable and uncountable, plural tarnations)
This New Englandism has fallen out of use in New England, but is remembered for its colorfulness and is still used in the Southeastern United States as a minced oath, where ‘hell’ or ‘damn’ would otherwise be said, especially in the phrase "what in tarnation".