What in Tarnation?

Origin

The word tarnation is a softened form of "dangnation." It appeared in **American English in the late 1700s**, especially in New England speech, and soon spread westward.

Early settlers, preachers, and writers used it as a way to avoid swearing - much like "Sam Hill," "golly," or "heck." By the 19th century, it was everywhere from Yankee tall tales to Southern storytelling, cowboy humor, and early comic strips.

Verdict: Southernish. Widespread across early American frontier speech, later adopted into Southern and Appalachian talk.