water under the bridge逝者如斯夫

无法重新来过或不值得再讨论的往事；另一种解释是很长时间过去了（a long time has passed）。

例：

I should probably have asked for more money when I was offered the job, but hey, that's water under the bridge now.

我拿到这份工作的时候，或许该要求更高的薪资来着。不过，嗨，现在说什么都晚了。