Here’s an interesting article on the BBC website about anti-languages, such as Thieves Cant, Polari and Gobbledygook.

They have been around at least since Tudor times, and are used as secret slang by prisoners, escaped slaves, criminal gangs and gay people to keep their activities secret from the police and other authorities.

Any group of people who spend a lot of time together tend to develop their own vocabulary and references which can be mystifying to outsiders, but these anti-languages are deliberately created to prevent outsiders from understanding.

Some anti-language words become part of mainstream language. For example, from Polari, a form of gay slang, we get camp, butch, palaver and naff.