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The sentence said by a boy who looks like being angry because of something.
なんでもねえやこっちのこったい! なんにしてもおれはでねえぞっ 一歩だってこの ひみつ練習場を...
I have found that several Japanese forums told that「なんてこったい」 means "What the hell" or "Oh my god" but I'm not sure what just the word alone really means.
こっちのこったい is a colloquial contraction of こっちのことだい, lit. It is a matter of my/our side, → "It's not your concern." "It's none of your business."
Compare:
なんてこったい！ → なんてことだい "What the hell?"
いやなこったい！ → いやなことだい (≂ いやだ) "No way!"
Ending a sentence with ～だい is Tokyo/Edo dialect (I think), e.g. 「行くんだい！」「何言ってんだい！」
なんでもねえや → It's nothing.
こっちのこったい! → Mind your own business.
なんにしても → In any case,
おれはでねえぞっ → I'll never get out (of this...)
一歩だって → not even a single step
このひみつ(の?)練習場を... → (... get out) of this secret gym / training room...
こっちのこったい is casual expression of こっちのことです
In this case, こったい is same meaning as ことです
こっちのこと means my own business
So こっちのこったい means it is my own business.
