vernacular (adj.)

c. 1600, "native to a country," from Latin vernaculus "domestic, native, indigenous; pertaining to home-born slaves," from verna "home-born slave, native," a word of Etruscan origin. Used in English in the sense of Latin vernacula vocabula, in reference to language. As a noun, "native speech or language of a place," from 1706.

For human speech is after all a democratic product, the creation, not of scholars and grammarians, but of unschooled and unlettered people. Scholars and men of education may cultivate and enrich it, and make it flower into the beauty of a literary language; but its rarest blooms are grafted on a wild stock, and its roots are deep-buried in the common soil. [Logan Pearsall Smith, "Words and Idioms," 1925]

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Definitions of vernacular from WordNet
vernacular (n.)
a characteristic language of a particular group (as among thieves);
Synonyms: slang / cant / jargon / lingo / argot / patois
vernacular (n.)
the everyday speech of the people (as distinguished from literary language);
vernacular (adj.)
being or characteristic of or appropriate to everyday language;
a vernacular term
vernacular speakers
Synonyms: common / vulgar