keeper

finders keepers(, losers weepers)

Prov. If you find something, you are entitled to keep it. (This is a children's rhyme and sounds childish when used by adults.) Bill: Hey! How come you're using my fountain pen? Fred: It's mine now. I found it on the floor—finders keepers, losers weepers. Child: That's my hat. You can't have it. Playmate: I found it. Finders keepers.

I am not my brother's keeper. and Am I my brother's keeper?

Prov. You are not responsible for another person's doings or whereabouts. (Biblical.) Fred: Where's Robert? Jane: Am I my brother's keeper? Jill: How could you let Jane run off like that? Alan: I'm not my brother's keeper.

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not be your brother's keeper  also not be somebody's keeper

to not be responsible for what someone does or for what happens to them It's all too easy for us not to intervene in another country's problems, telling ourselves that we're not our brother's keeper. You shouldn't blame yourself for what's happened to Simon. You're not his keeper, you know.

Finders keepers (losers weepers).

something that you say when you find something that belongs to someone else and decide you are going to keep it 'Finders keepers,' he said, putting the money away in his pocket.