From Latin 'vernaculus' (domestic), from 'verna' (home-born slave) — shifted to 'ordinary people's language.'
The language or dialect spoken by the ordinary people of a country or region; (as adjective) of or relating to the native language or dialect of a place; domestic, native, not foreign.
From Latin 'vernāculus' (domestic, native, indigenous, born in the house), from 'verna' (a home-born slave, a slave born in the master's household as opposed to one bought abroad), a word of uncertain origin — possibly Etruscan, as it has no clear Indo-European etymology. 'Verna' and its adjective 'vernāculus' carried connotations of being native to a place, of belonging to the local household rather than being imported. In rhetoric, 'sermo vernāculus' was the ordinary speech of native inhabitants — the language of daily life
Dante Alighieri's 'De Vulgari Eloquentia' (On Eloquence in the Vernacular, c. 1303) was one of the first defenses of writing in a vernacular language rather than Latin. Ironically, Dante wrote this defense of the vernacular in Latin — because Latin was still the only language in which a serious scholarly argument would be taken seriously. He had to use the prestige language to argue for the legitimacy of the common one.