late 14c., "common, ordinary," from Latin vulgaris, volgaris "of or pertaining to the common people, common, vulgar, low, mean," from vulgus "the common people, multitude, crowd, throng," perhaps from a PIE root *wel- "to crowd, throng" (source also of Sanskrit vargah "division, group," Greek eilein "to press, throng," Middle Breton gwal'ch "abundance," Welsh gwala "sufficiency, enough") [not in Watkins]. Meaning "coarse, low, ill-bred" is first recorded 1640s, probably from earlier use (with reference to people) with meaning "belonging to the ordinary class" (1530). Chaucer uses peplish for "vulgar, common, plebeian" (late 14c.). Related: Vulgarly.
What we have added to human depravity is again a thoroughly Roman quality, perhaps even a Roman invention: vulgarity. That word means the mind of the herd, and specifically the herd in the city, the gutter, and the tavern. [Guy Davenport, "Wheel Ruts"]
For Vulgar Latin, see here.