"Down the drain" is an English idiom that means that something has either been lost or wasted. The meaning of the idiom comes from the fact that if something were literally to go down some a drain, the object would not be possible to recover. There are many different ways in which "down the drain" is used, although it is often associated with the loss of money. In other cases, it may refer to hopes or dreams wasted or effort that comes to naught.
Any time that a person uses a phrase that has a meaning somewhat different from its literal meaning, the phrase is known as an idiom. Idioms are common in the English language, and they gain their meanings through constant use in everyday life. People learn the meanings through usage by others, until the phrase takes on a meaning all its own separate from its literal meaning. One such idiom is the phrase "down the drain," one of the most popular phrases used to indicate that something has been lost.
People often use this idiom when they are discussing a sum of money that has been either lost or wasted. For example, a person might say, "We paid so much for that TV and it never worked right; that's money down the drain." The intent of the idiom here is to convey the fact that the money paid for the television has been completely wasted.
On other occasions, people use this particular idiom to indicate that something precious has been lost and can't be recovered. As an example, imagine someone saying, "Things are never going to work out for me, so all my hopes and dreams are going down the drain." The intent of the idiom in this case is to indicate that the speaker doesn't think he will ever fulfill the hopes and dreams that he mentions. Thus, they are essentially wasted for him.
This idiom can also used in the context of wasted effort. Imagine someone saying, "All that work we put into it went right down the drain." In this case, the person wants to convey that whatever work that has been done will not come to any fruition due to some circumstance. The exaggeration inherent in the idiom gives the phrase its color and meaning.