Word of the day: argot
Argot is a specialised idiomatic vocabulary peculiar to a particular class or group of people, especially that of an underworld group, devised for private communication and identification.
I guess classic examples would be the language used by criminal gangs or prison inmates, but the term argot is used more widely to refer to specialised jargon largely unintelligible to a lay audience.
I came across the term in the Hillary Clinton memoir where she uses the word rather cynically to refer to the sort of language used by many in the diplomatic community.