argot (n.)

1860, from French argot (17c.) "the jargon of Paris rogues and thieves" (for purposes of disguise and concealment), earlier "the company of beggars," from Middle French argot, "group of beggars," a word of unknown origin.

Gamillscheg suggests a connection to Old French argoter "to cut off the stubs left in pruning," with a connecting sense of "to get a grip on." The best English equivalent is perhaps cant. The German equivalent is Rotwelsch, literally "Red Welsh," but the first element of that might be connected with Middle High German rot "beggar." Compare pedlar's French (1520s) "language of thieves and vagabonds."