The origin of common expressions is always a fascinating subject. Some origins are reasonably obvious, like “flash in the pan” and “going off half-cocked,” come from firearms. But other derivations are more obscure.
Take the word “tarnation.” According to the Oxford English Dictionary, this expression is a combination of two terms: darnation, a natural variant of damnation; and tarnal, a pronunciation of eternal that was intended to be a mild curse. Thus, it’s just a milder way to say damnation.
The earliest written reference to the term was found in a 1787 play called The Contrast: A Comedy in Five Acts. Written by Royall Tyler and set in New York City, the play satirized two sects in American society: snooty Europhiles and hardy patriots. It’s considered by many to be this country’s first theatrical comedy.
Though often associated with the Wild West, the expression wasn’t confined to that region or any particular era. Instead, consider tarnation as a sense of homespun Americana.
Taken from “What in Tarnation is Tarnation?” by Ellen Gutoskey at (https://www.mentalfloss.com/posts/what-in-tarnation-meaning?)