Argot

Argot \AHR-goh\ or \AHR-guht\ , noun;
1. A specialized idiomatic vocabulary peculiar to a particular class or group of people, especially that of an underworld group, devised for private communication and identification
2. The special vocabulary and idiom of a particular profession or social group

This word dates to 1860 from French argot which meant "the jargon of Paris rogues and thieves" and earlier "the company of beggars," which derives from the Middle French definition "group of beggars." Before that the origin is unknown. The best English equivalent is cant, which dates to 1709 as "insincere talk" from the earlier slang definition "whining of beggars." It derives from Old Northern French canter ("to sing, chant"), which comes from Latin cantare, related to canere ("to sing").