Water under the bridge is used to say that something happened in the past and is no longer important or worth arguing about.

Note: Water over the bridge and the expression “let bygones be bygones” can be synonyms. However, “let bygones be bygones” encourages the desire to forgive and forget, not hold a grudge. (See the post ‘let bygones be bygones.’)

For example:

We had our differences in the past, but that’s all water under the bridge now.

Lily was relieved her divorce was over and regarded it as water under the bridge.

Let’s consider last year’s project failures as water under the bridge.

Tradução: coisas do passado, águas passadas, águas passsadas não movem moinhos.