Definition: something in the past; something that is over, finished, forgotten
We had a big fight last year, but we are good friends again. I don’t even remember what the fight was about — it is water under the bridge.
I made a huge mistake last month, and we almost lost a big customer. However, I quickly corrected the mistake, kept the customer, and my boss forgave me. Yesterday he said don’t worry, it is water under the bridge.