The Collaborative International Dictionary of English v.0.48:

Argot \Ar`got"\, n. [F. Of unknown origin.]
   A secret language or conventional slang peculiar to thieves,
   tramps, and vagabonds; flash.
   [1913 Webster]

WordNet (r) 3.0 (2006):

argot
    n 1: a characteristic language of a particular group (as among
         thieves); "they don't speak our lingo" [syn: slang,
         cant, jargon, lingo, argot, patois, vernacular]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "argot":
   Aesopian language, Babel, Greek, babble, cant, cipher, code,
   cryptogram, double Dutch, garble, gibberish, gift of tongues,
   glossolalia, gobbledygook, jargon, jumble, lingo, mumbo jumbo,
   noise, patois, patter, phraseology, scatology, scramble,
   secret language, slang, taboo language, vernacular, vocabulary,
   vulgar language