Source : Webster's Revised Unabridged Dictionary (1913)

Patois \Pa`tois"\, n. [F.]
   A dialect peculiar to the illiterate classes; a provincial
   form of speech.

         The jargon and patois of several provinces. --Sir T.
                                                  Browne.

Source : WordNet®

patois
     n 1: a characteristic language of a particular group (as among
          thieves); "they don't speak our lingo" [syn: {cant}, {jargon},
           {slang}, {lingo}, {argot}, {vernacular}]
     2: a regional dialect of a language (especially French);
        usually considered substandard