the definition for 一体【いったい】:
Adverb : ...the heck (e.g. "what the heck?"); ...in the world (e.g. "why in the world?"); ...on earth (e.g. "who on earth?");
—Before an interrogative, forms an emphatic question. 一体どこで彼に会ったんだ。Where on earth did you meet him? いったいなぜ彼を駅に連れて行ったの？Why on earth did you take him to the station?
Noun : one object; one body; unity
one form; one style one Buddhist image (or carving, etc.)
Adverbial noun :
generally; in general; → 一体に
Given the definition of the noun, why does this word possess an implied incredulity when used before an interrogative?