The American Heritage® Dictionary of the English Language, 4th Edition
adj. Chiefly New England & Upper Southern U.S. Damned. See Regional Note at tarnation.
The Century Dictionary and Cyclopedia
An epithet of reprobation: used as a piece of mild profanity.
Word Usage
""Boys, I'm drinking, and I don't care, fur my spunk's up, and I'd just's soon bet her all the first bet; them tarnal fellers guzzled me out of $1,000 in Cincinnater, and I wants ter get even.""