What in tarnation is a tarnation? Dagnabbit!
Good question! I've always thought this: It's a North Carolina saying.
N.C. is known as the Tarheel state. I use to hear my parents say it in place of other "choice" words.
Dagnabbit, is a good one too!
Update: I looked this up.
tar·na·tion (tär-nshn) New England & Southern U.S. n. The act of damning or the condition of being damned. interj. Used to express anger or annoyance.
http://www.thefreedictionary.com/tarnation
Here's a further explanation:
Question - What is the etymology of the word "tarnation" as in "what in tarnation?" Is it a melding of "entire nation?"
Answer - In early English, the word 'tarnal' evolved as a vernacular form of 'eternal' . Eternal damnation was shortened, through slang usage, to tarnation and tarnation became a gentle replacement for the word damnation when said in anger or frustration. The following is what Morris' Dictionary of Word and Phrase Origins has to say about 'tarnation': In the days when our country was young, men were very careful not to use even mild profamity in front of women. So "Damnation," a heartfelt expression of exasperation or disgust , was toned down a bit and became "Tarnation"
I really thought is was just some terminology specific to N.C.! Ha!