water under the bridge 泼出之水；不可改变的既成事实；无法挽回的过去

【例句】
All that is now just water under the bridge.
所有那些现在都是不可改变的事实了。

At this point, that problem is just water under the bridge. it's not important any more.
从这点上看，这个问题已经无关紧要了，不再是什么问题了。