It is etymologically appropriate that the term mob should be associated with a roiling crowd, because the word is a clipped form of mobile.
That word, in Latin, means “movable,” but it also had the sense of “fickle,” which was what is meant by the Latin phrase mobile vulgus, which refers to the perception of the ruling class in ancient Rome that public opinion was capricious. (Plus ça change . . .)
(Vulgus, meanwhile, means “the common people,” and the perceived base behavior of the rabble—again, judged from above—prompted the adjective vulgar. The Latin term also begat another adjective, the first word in the phrase “Vulgate Bible,” referring to a translation of the Bible accessible to the populace).
The slang shortening of mobile to mob occurred sometime in the late 1600s, and we still use it to refer to a large, unruly group of people. (Mobcap, the word for a large woman’s cap worn indoors, is unrelated; it comes from the obsolete name Mab.) To mob someone originally meant to attack him or her in a group; it now applies to any mass of people accosting someone, as when a celebrity is spotted in a public place and besieged for autographs or to be photographed.
Mob is also associated with organized crime during the Roaring Twenties (at about the same time that the phrase “mob scene” was coined to refer to a crowded place), but it had first been applied to a gang of criminals nearly a hundred years earlier than that. Mobster was first attested in 1916, about twenty years after gangster officially entered the lexicon.
The adjective mobile means “able to move or be moved.” (The name of the city of Mobile in Alabama is unrelated; it derives from a Native American word.) In the 1930s, the word was first used to modify the noun sculpture to refer to a piece of art, usually suspended, in which motion is integral to the effect of the art on the observer; in the late part of the following decade, the adjective stood on its own to become a noun referring to such art.
Automobile was first, in the mid-nineteenth century, an adjective (a mash-up of Greek and French-based-on-Latin meaning “self-moving”); the French phrase véhicule automobile was truncated in the late 1800s to automobile, which briefly had in French the synonym locomobile (loco means “from a place,” hence locomotive, “moving from a place”). During the transitional period when use of horse-drawn vehicles and early automobiles coincided, the term hippomobile (the first element is from the Greek word for horse, known mainly from hippopotamus, or “water horse”) distinguished the former from the latter. Snowmobiles, developed in the early 1900s, were so named starting in 1931.
Mobile homes, derived from travel trailers and originally designed early in the twentieth century to accommodate people who needed to move often, later developed into prefabricated homes that could be hauled to a permanent or semipermanent location, resulting in the name being oxymoronic.
Mobility is the quality of being mobile, while to mobilize is to make capable of movement; the military sense, which refers to a country’s large-scale preparation for war, actually precedes the general definition. (It was first used in the mid-eighteenth century.) The noun form of mobilize is mobilization.