覆水难收

fùshuǐ-nánshōu

[spilt water can't be gathered up; what is done cannot be undone; it is hard to get it back again; water poured on the ground cannot be recovered] 倒在地上的水难收回来。 比喻事成定局, 无法挽回(多用于夫妻离异)

成媒不说, 覆水难收。 --《敦煌变文集》