Water under the bridge is an old quarrel that no longer matters. It is all water under the bridge now.
It is said to close a subject rather than to settle it, which is why it can be generous or dismissive. The picture is of something that has already flowed past and cannot be reached.
1phrasewater under the bridgeAn old quarrel that no longer matters.
“Whatever was said in 2011 is water under the bridge.”
water under the bridge is a phrase.
Practised in Idioms