According to Wiktionary, the English term "mob" (as in group of people) comes from the Middle English "mobile", which comes from the Latin "mobile (vulgus)" (a moving crowd). Is this meaning attested in Classical Latin? here we can find other Classical Latin words for "mob", like turba, vulgus, [promiscua] multitudo and globus. But what about mobile vulgus, from where "mob" is supposed to come?
The phrase "mobile vulgus" occurs in Seneca and in Statius. See here:
Mobile vulgus occurs sometimes in Classical Latin, and among the main authors that use this phrase are:
Publius Papinius Statius (1st century AD);
Seneca in his Hercules Furens (vv.169-171):
"Illum populi favor attonitum
fluctuque magis mobile vulgus
aura tumidum tollit inani".
Also Tacitus comments on the mobilitas vulgi in Hist. V 8,3.
But an interesting reference (my favourite) also occurs in a Latin translation of the major work of a well known Greek poet who lived in the 3rd century BC! We are speaking of Aratus, and of his Phenomena, "appearances":
It would be even more interesting to dig into the Greek original text and search for the exact expression used by the author.