Most commonly found at the bottom of sinks and bathtubs across the globe, a drain is the hole through which water is carried away.
If it rains on the day you decided to go to the beach, you could say your plans went right "down the drain." When used as a verb, to drain means "to gradually empty," whether it's a man-made lake that's got to go or some financial obligations putting a strain on your wallet. And that obnoxious guy who's sucking the life out of the party? He's a real drain too.