1688, "disorderly part of the population, rabble," slang shortening of
mobile, mobility "common people, populace, rabble" (1676), from L.
mobile vulgus "fickle common people" (1600), from
mobile, neut. of
mobilis "fickle, movable, mobile," from
movere "to move" (see
move). In Australia and N.Z., used without disparagement for "a crowd." Meaning "gang of criminals working together" is from 1839, originally of thieves or pick-pockets; Amer.Eng. sense of "organized crime in general" is from 1927. The verb meaning "to attack in a mob" is attested from 1709.
Mobster is first attested 1917.
Mob scene "crowded place" first recorded 1922.
Mobocracy "mob rule" is attested from 1754.