FKK is an abbreviation that is found in several european languages, it originaly stems from german and is short for "frei-körper-kultur" (free body culture). it can be found on signs at beaches and lakes, tellng the visitor that he is going to enter an area where it is common to swim and sunbath nude.

fkk does not always mean the same as "clothing optional", sometimes the only accepted way to bath there is in your bithday suit :) whereever you find such a sign there is a regular beach around somewhere

https://www.urbandictionary.com/define.php?term=FKK

https://en.wikipedia.org/wiki/Freik%C3%B6rperkultur