낙장불입이에요 · 낙장불입이야 (casual)
no take-backs, what's done can't be undone
Example
한번 낸 카드는 낙장불입이니까 다시 가져갈 수 없어요.
Once a card is played, there are no take-backs, so you can't pick it up again.
Word parts
낙 fall+장 sheet, card+불 not+입 enter
Literally 'a dropped card doesn't come back in' — a rule from the card game hwatu.
Good to know
Used jokingly beyond games, meaning a decision or statement can't be reversed.