Question

Idom meaning is 'water under the bridge'

Answer 1

That's happened in the past, cannot be undone, and can no longer be a consideration. The proverb comes from the saying: 'A lot of water has flowed (passed, gone) over the dam (under the bridge.) 'Under the bridge' is British and is the oldest part of the proverb. 'Over the dam' is its American variant