Definition & Meaning of "what in tarnation"in English

What is the origin of the exclamation "what in tarnation" and how is it used?

What in tarnation is an exclamation used to express shock, confusion, or disbelief. It appeared in American English in the 19th century, mainly in rural and Southern speech, as a euphemistic alternative to invoking God or the devil. The phrase uses tarnation as a softened, folk-style swear word. Today it sounds rustic, humorous, or old-fashioned, and is mostly used for comic or emphatic effect rather than serious anger.