The Wordery: Retronym
Found this one actually used on Wikipedia. It’s the term for any out-of-date slang name for a certain sub-culture, demographic etc. A good example may be if you were to refer to a hipster as a “beatnik.” This came up as a UK-based friend referred to local hooligans as “neds.” It’s a nice little post-language word of its own. Read more about Neds HERE.
UPDATE FROM CHRIS SWANSON via AIM:
“it’s an acronym that is not based not on the actual logic that created it, but rather on a convenient acronym that was decided upon after the fact, like when the religious right said that KISS stood for Knights in Satan’s Service, or AC/DC was After Christ Devil Comes.”
You see? The fun thing about thing about post-language is everyone can join in.