If you want to fill a sink or bathtub with water, you need to do one thing first and that's stopper or plug up the drainpipe.
如果你想把水槽或浴缸放满水，你需要做的第一件事这就是用塞子之类的东西堵住排水管。
Later, when you unplug the stopper, the used water will go rushing out down the drain.
然后，当你拔掉塞子之后，用过的水就会漏进下水道里。
Hopefully, nothing valuable (such as a diamond ring) will be in that water, because after the water flows down the drain, it is gone forever.
我们都希望不要有什么贵重的东西（比如钻戒）掉在水里——因为水流进下水道之后，这些贵重东西永远也找不回来了。
Anything that was mistakenly dropped into the water will therefore also disappear down the drain, and then it will be very difficult probably impossible to get it back.
任何掉在水里的东西将最终落入下水道里——这将非常困难——恐怕不可能——把它找回来了。
From the sad reality of losing a cherished object down a sink or bathtub drain comes this idiom, meaning to do or attempt to do something that ultimately will be lost forever.
通过这个直观的解释，我们理解了这个成语。它的意思是“做或试图做一些最终将永远失去的事情”。
It can also mean to put great effort into a wasted activity.
同时，它还含有“将巨大的努力浪费在无用的事情上”的意思。