The word tarnation was coined in 1784 in New England, but its use in the Northeast was quickly superseded by that of the South. This is sort of a mutt of words. It’s a bowdlerizing alteration of the word darnation, but it’s sort of influenced by the word eternal, kind of emphasizing eternal darnation, the latter word of which is a euphemism for damnation, so this is sort of a double euphemism. The –ation part is just a suffix, but damn, through French, comes to us from Latin damnum, which meant "loss", as in loss of salvation. Through Proto-Italic dapno, this is from Proto-Indo-European dehp, meaning "to sacrifice" or still "lose". Eternity isn't quite the etymon in tarnation, only affecting its development through influence, so I'll save that for a future post. As a word, tarnation was used most frequently in the 1800s, but there's been a recent increasing period due to its whimsical usage on the Internet.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.