Argot
Etymology
From French argot, first recorded in the seventeenth century for the jargon of thieves and beggars; its ultimate origin is unknown. The secrecy was the point: an argot let the underworld talk business in the open. The term later broadened to any group’s in-crowd vocabulary, though the flavor of exclusion clings to it.
noun · /AR-goh/
The private vocabulary of a particular group, especially one used to keep outsiders out.
What it names
Language as a membership card — the coded slang of a subculture that both bonds insiders and screens out anyone who doesn’t belong.
Example
Skaters have an argot so dense that a bystander catches maybe one word in five.
Related idea
Argot carries a whiff of secrecy and closed circles, while vernacular is simply the ordinary everyday speech of a place — open to all, hiding nothing.