water under the bridge 是什么意思

　　water under the bridge

　　已成往事，不可改变的既成事实

　　water 是“水”，under the bridge 是“在桥下”，那这个表达是指“水在桥下”?“桥下的水”吗?

　　water under the bridge 是个习语，其释义为：problems that someone has had in the past that they do not worry about because they happened a long time ago and cannot now be changed 既成事实，已成往事

　　eg. We had our differences in the past, but that's all water under the bridge now.