What does water under the bridge mean?

Answer 1

"Water under the bridge" means that you are willing to forget something and move on in life. Like a drop of water flowing under a bridge, that point in time has already passed and will not reoccur. You cannot change what happened, so it's best to quit dwelling on the past and move on.

Is an adage meaning something is past and done with.

The phrase 'water under the bridge' means past events to be put aside.

The origin of the phrase comes from ancient Greek Heraclitus who said "You could not step twice into the same river". It means that the river is ever changing because of the flow of the water, the river is never the same a second later. So what happened in the past should stay in the past and people can change and become better persons. A synonym could be: "Forget and forgive"

"It's all water under the bridge" means this -- Even though bad things happened in the past, none of it matters anymore. Times have moved on. Here's an example: Deanie Etcetera was told by Mitch's relative that Mitch wasn't interested in her. For a long time afterward, Deanie was absolutely devestated, both by Mitch for not being interested, and also by Mitch's relative for telling her such hurtful news in the first place ! Nowadays, Deanie doesn't really care about any of that anymore. "It's all water under the bridge".
Bygones, basically. If you're referring to an old matter as water under the bridge, you're not making it a concern anymore and wish to let it go like the water that flows under the bridges and is soon out of sight so you can turn to other matters.