I've come across something and always wondered about it, what is the meaning of
"なんてこったいこりゃいったい"?
I assume (but I'm not quite sure) that it's this: to show disbelief or surprise
Also, is this something actually used? (and if it is, in what context?)
なんてこったいこりゃいったい
is a colloquial, slurred pronunciation of 「なんてことだい、これはいったい」
which is a reversed version (倒置) of 「これはいったい、なんてことだい?」
To break it down...
なんて -- pronoun [何]{なん} + case particle (っ)て; ≂「なんという」
こったい -- noun こと (事) + copula だ + sentence ending particle い (≂よ)
こりゃ -- pronoun これ + particle は
いったい -- noun 一体 (used for emphasis in questions; "on earth")
So it literally means... "What kind of thing on earth is this?" "What on earth is the matter?" -- used as an exclamation, like "What the hell is this?"
Yes it is, but there your text has two parts:
なんてこった (い)
which is an exclamation that roughly translates to "oh no", and
こりゃいったい
which means "just what (is this)...".