If you say that an event or incident is 'water under the bridge', you mean -

Water under the bridge (phrase)
English meaning: Used to refer to events that are in the past and consequently no longer to be regarded as important.
Bangla Meaning: যে ঘটনা বা বিষয় অতীতে ঘটে গেছে এবং এখন আর তা নিয়ে চিন্তা করার বা দুশ্চিন্তা করার প্রয়োজন নেই।
- If you say that an event or incident is water under the bridge, you mean that it has happened and cannot now be changed, so there is no point in worrying about it any more.

Example:
- I don't want to talk about that—it's all water under the bridge now.
- He was relieved his time in jail was over and regarded it as water under the bridge.

Source:
1. Oxford Learner's Dictionery.
2. Collin's Dictionary.