(All) water under the bridge (AmE: water over the dam)
Something that happened in the past and cannot now be changed or is already forgotten, so it is not important any more.
“We used to have terrible quarrels but that’s all water under the bridge now.”
Origin
This proverb probably relates to the fact that water of a river flows constantly and never returns back. It was first recorded at the beginning of the 20th century.
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