c. 1300, "the people of a country, a community," from Old French comunalte, from comun (see common (adj.) as if from Medieval Latin *communalitas. A respelling of commonalty (late 13c.). Meaning "the common people" is attested from 1580s; that of "state or quality of being shared" is from 1954.
late 14c., "common, ordinary," from Latin vulgaris, volgaris "of or pertaining to the common people, common, vulgar, low, mean," from vulgus, volgus "the common people, multitude, crowd, throng," for which de Vaan offers no further etymology.
The meaning "coarse, low, ill-bred" is recorded by 1640s, probably from earlier use (with reference to people) in the meaning "belonging to the ordinary class" (1520s). 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.
in writing, to indicate the common casual pronunciation of could have, by 1909.