finders keepers (losers weepers)used to say that if someone finds something, they have the right to keep it
[TahlilGaran] Dictionary of Contemporary English ▲
finders keepers informal Those who find lost things can keep them.
■ Used usually by children to claim the right to keep something they have found.
I don't have to give it back; it's finders keepers. Finders keepers, losers weepers! It's my knife now! [TahlilGaran] English Idioms Dictionary ▲