I am always amazed at how quickly wors can morph into completely different meanings, e.g. "nuke" comes from nuclear, and the initial phrase to "nuke 'em" meant to use nuclear weapons on another country. This has now morphed into all kinds of uses, for instance in sports if one team wins by a large margin they say they "nuked" the other team.

Another one is "gay." First it simply meant cheerful, then it meant homosexual, but now among twenty somethings it means it can means something stupid, as in "that idea is so gay."