The Collaborative International Dictionary of English v.0.48:

Patois \Pa`tois"\ (p[.a]`tw[aum]"), n. [F.]
   A dialect peculiar to the illiterate classes; a provincial
   form of speech.
   [1913 Webster]

         The jargon and patois of several provinces. --Sir T.
                                                  Browne.
   [1913 Webster]

WordNet (r) 3.0 (2006):

patois
    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]
    2: a regional dialect of a language (especially French); usually
       considered substandard

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "patois":
   Acadian, Anglo-Indian, Brooklynese, Cajun, Canadian French,
   Cockney, French Canadian, Gullah, Midland, Midland dialect,
   New England dialect, Pennsylvania Dutch, Yankee, Yorkshire, argot,
   bundle of isoglosses, cant, class dialect, colloquial, dialect,
   dialect atlas, dialect dictionary, gibberish, gobbledygook, idiom,
   isogloss, jargon, lingo, linguistic atlas, linguistic community,
   linguistic island, local dialect, localism, mumbo jumbo, patter,
   phraseology, provincialism, regional accent, regionalism,
   scatology, slang, speech community, subdialect, taboo language,
   vernacular, vocabulary, vulgar language, vulgate