"passage from the mouth of an animal to the stomach," c. 1300 (as a surname), from Old French golet "neck (of a bottle); gutter; bay, creek," diminutive of gole "throat, neck" (Modern French gueule), from Latin gula "throat," also "appetite," which is related to gluttire "to gulp down, devour," glutto "a glutton." De Vaan writes, "We seem to be dealing with an onomatopoeic formation of the form *gul- / *glu-." Compare Old English ceole "throat;" Old Church Slavonic glutu "gullet," Russian glot "draught, gulp;" Old Irish gelim "I devour."
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.