A 'drain' is an area where wastewater is disposed of. If you say that something 'has gone down the drain' you mean that it is lost or has been wasted needlessly. This expression is usually used to talk about money or resources that were used ineffectively. This is an informal idiom.

1. When my science project dropped and broke, I thought, Great! All that hard work down the drain!
2. Every weekend I go to the pub, and all my money goes down the drain.

1. During the meeting, all my great ideas went down the drain as people suggested them before I got a chance.
2. If we don't have intelligent marketers as well as a great campaign, all of our advertising budget might as well go down the drain.