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Phrase(s): 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.
McGrawhill's American Idioms And Phrasal Verbs