Definition of water under the bridge in English English dictionary
Something in the past that cannot be controlled or undone, but must be accepted, forgiven, or forgotten
They agreed that their old disputes were water under the bridge and decided to make a fresh start.
(deyim) Problems that someone has had in the past that they do not worry about because they happened a long time ago and cannot now be changed: "Yes, we did have our disagreements but that's water under the bridge now."
(deyim) The past, history. -That failure is water under the bridge. Let's not live in the past