mobster n.

1680s, "disorderly part of the population, rabble, common mass, the multitude, especially when rude or disorderly; a riotous assemblage," slang shortening of mobile, mobility "common people, populace, rabble" (1670s, probably with a conscious play on nobility), from Latin mobile vulgus "fickle common people" (the Latin phrase is attested c. 1600 in English), from mobile, neuter of mobilis "fickle, movable, mobile" (see mobile (adj.)).

Mob is a very strong word for a tumultuous or even riotous assembly, moved to or toward lawlessness by discontent or some similar exciting cause. Rabble is a contemptuous word for the very lowest classes, considered as confused or without sufficient strength or unity of feeling to make them especially dangerous. [Century Dictionary, 1897]

Also used of a promiscuous aggregation of people in any rank of life (1680s), and in Australia and New Zealand used without disparagement for "a crowd." Meaning "gang of criminals working together" is from 1839, originally of thieves or pick-pockets; the American English sense of "organized crime in general" is from 1927.

The Mob was not a synonym for the Mafia. It was an alliance of Jews, Italians, and a few Irishmen, some of them brilliant, who organized the supply, and often the production, of liquor during the thirteen years, ten months, and nineteen days of Prohibition. ... Their alliance — sometimes called the Combination but never the Mafia — was part of the urgent process of Americanizing crime. [Pete Hamill, "Why Sinatra Matters," 1998]

Mob scene "crowded place" is by 1922, from earlier use in reference to movies and theatrical productions; mob-rule "ochlocracy" is by 1806.

Old English -istre, from Proto-Germanic *-istrijon, feminine agent suffix used as the equivalent of masculine -ere (see -er (1)). Also used in Middle English to form nouns of action (meaning "a person who ...") without regard for gender.

The genderless agent noun use apparently was a broader application of the original feminine suffix, beginning in the north of England, but linguists disagree over whether this indicates female domination of weaving and baking trades, as represented in surnames such as Webster, Baxter, Brewster, etc. (though modern spinster probably carries an originally female ending).

Also compare whitester "one who bleaches cloth;" kempster (c. 1400; Halliwell has it as kembster) "woman who cleans wool." Chaucer ("Merchant's Tale") has chidester "an angry woman" (the 17c. had scoldster). Also compare Middle English shepster (late 14c.) "dressmaker, female cutter-out," literally "shapester," sleestere (mid-15c.) "murderess, female killer" ("slay-ster"). Sewster "seamstress" (Middle English seuestre, late 13c. as a surname, also used of men) is still in Jonson but was obsolete or provincial after 17c. In Modern English, the suffix has been productive in forming derivative nouns (gamester, roadster, punster, rodster "angler," etc.).