Patois, n.
Etymology: [F.]
A dialect peculiar to the illiterate classes; a provincial form of speech. The jargon and patois of several provinces. Sir T. Browne.
Shades of meaning
Fernald has no entry of its own for patois; he lists it under language.
Where he writes about it:
A dialect may be used by the highest as well as the lowest within its range; a patois is distinctly illiterate, belonging to the lower classes; those who speak a patois understand the cultured form of their own language, but speak only the degraded form, as in the case of the Italian lazzaroni or the former negro slaves in the United States. — from language
Antonyms
none given