A euphemism is a nice way of saying something not so nice. The problem with euphemisms is that the more successful they are, the more likely they are to take on the not so nice meaning of the thing they're trying to be nice about. Once this reversal happens, they transition from a euphemism to a synonym and a new euphemism must be created. It's an interesting quirk of language that a word or phrase can be intended to mean one thing and then become so popular that it comes to mean something entirely different.
