It stands for Frei Korper Kultur (free body culture), which is the German name for the subculture that is into nudism. A bunch of brothels (by now, quite a lot of them) use the abbreviation to indicate a sexclub that offers wellness options and (in most cases) have girls walking around naked.

The clubs really don't have anything to do with nudism, but they use the abbreviation. I'm sure actual nudists aren't too happy about it, since nudism is explicitly not about sex.