Fetching details from the dictionary...

other people's affairs; someone else's problem
彼はいつも他人ごとのような顔をしている。
He always has an expression like it's none of his concern.
地震の備えを他人ごとだと思わないでほしい。
I want people not to treat earthquake preparedness as someone else's problem.
環境問題は決して他人ごとではない。
Environmental issues are by no means someone else's concern.