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なっちまった

ended up becoming

→ 成る
なっ become; come to be + ちまっ Completed action (colloquial) + た Past tense
verb · past indicative Rare #35,138
ended up becoming A very casual contraction of なってしまった, often expressing regret, resignation, or that something happened completely.
JA: いつの間にか、大人になっちまった。
EN: Before I knew it, I had ended up becoming an adult.
Conjugation of 成る

まったくどうして･･･どうしてこんなことになっちまったのかだからもういいって

Damn it, why... why did it come to this? I said, that's enough already.

— ドライヴ (2011)

まったく妙なことになっちまったぜ

This has turned into a real strange mess.

— 紅の豚 (1992)

あんたもいい男になっちまったねえ

You’ve grown into quite a handsome man.

— 紅の豚 (1992)

そのうちに味方はおれだけになっちまった

Before long, I ended up being the only one on his side.

— 紅の豚 (1992)

俺はどうなっちまったんだ

What the hell has happened to me?

— Carandiru (2003)