What In Tarnation Meaning

(idiomatic) used to add emphasis to "what" when beginning question.

Example: 2003. Judith Silverstein, Michael Lasky, "Online dating for dummies"
  What in Tarnation to Write I still remember the first girl I called up for a date at age 15.
2005. Charles Harrington Elster, "What in the word?", Page 35
  Other euphemisms of this type (and there are many) include what in tarnation for what in damnation and what the Sam Hill for what the damn hell.
2007. Diane Stanley. "The Mysterious Case of the Allbright Academy" Page 141.
  â€œWhat in tarnation do you think you're doing?”
  What in tarnation have you done with my wallet!?