nown
the language used by a particular type or group of people : an often more or less secret vocabulary and idiom peculiar to a particular group
He has been bombarded by thousands of scathing messages—known as being “flamed” in the argot of cyberspace. —Peter H. Lewis
The guild has an argot that they use for communication in public and secret. One can’t take an overheard conversation as meaning exactly what was said in the common tongue.
Ah, this one’s on many of the lists, you’ll probably see it in literature, it can be used to adjust the vocabulary of a NPC, it helps with faction building, so many uses! It’s definately a neat word for how it could affect your game.