Used as an intensifier after a question word (who, what, where, when, why, and how) to express extreme confusion, surprise, or aggravation. Primarily heard in US. And just how in tarnation am I supposed to have three reports done by 9 AM tomorrow? Where in tarnation did you find that rusty old car? Who in tarnation is making all that noise?
what in tarnation
An exclamation used to emphasize surprise, shock, anger, disgust, etc. Just what in tarnation is going on here? Aw, what in tarnation! I just had this fixed, and now there's a dent in it! What in tarnation! Don't sneak up on me like that!