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other people's affairs; someone else's problem
災害のニュースを他人事だと思ってはいけない。
You shouldn't treat disaster news as someone else's problem.
彼は友人の悩みを他人事のように聞いていた。
He listened to his friend's troubles as if they didn't concern him.
環境問題は他人事ではなく自分の問題だ。
Environmental issues are not someone else's problem — they're yours.