Water under the bridge

From Fresh Dictionary

English

Noun

water under the bridge

1. ((idiomatic{{#if:|, {{{2}}}{{#if:|, {{{3}}}{{#if:|, {{{4}}}{{#if:|, {{{5}}}{{#if:|, {{{6}}}{{#if:|, {{{7}}}{{#if:|, {{{8}}}{{#if:|, {{{9}}}}}}}}}}}}}}}}}}})) Something in the past that cannot be controlled or undone, but must be accepted, forgiven, or forgotten.

   They finally agreed that their old arguments were water under the bridge and decided to make a fresh start.