What is the origin of the phrase 'what in tarnation'?

Answer 1

Tarnation and darnation (the latter probably having come first) are both euphemistic forms of damnation. Tarnation seems to have been influenced by tarnal,another mild oath derived from (e)ternal! The Oxford English Dictionary cites late-18th-century examples of tarnation from New England, indicating that it has been part of American speech since colonial days.

http://www.thefreedictionary.com/tarnation

It's a trudication of " what in the entire nation?" Jus shows ya wut kin hapin to a langwidge over tym! ;)