I’m from southern Ohio. Throughout my growing up here, I’ve heard the word tarnation slung around, as well as used it quite a lot myself. For example (or e.g., now that I know how to use the abbreviation correctly):
Just what in the tarnation do you think you’re doing?
Or a shorter expression, which conveys confusion or dismay about something that’s happening or happened:
What in the tarnation?
It was only today that, after saying something with the word tarnation in the sentence, that I realized I didn’t know what it meant. So, of course, off I went to the OED. The word tarnation is a variant of darnation, which is a variant of damnation. If I fill in damnation where I would normally say tarnation, the result sounds surprisingly weird to me:
What in the damnation are you doing?
Odd. Very odd.