The suffix -ster originated in Old English as -ister, serving to turn a verb into an agent noun, one describing a person who (or thing that) performs an action. Beginning its linguistic life as the feminine equivalent of -er, it survives in the specifically feminine noun spinster—originally, “a woman who spins,” but now a label for an older unmarried woman.
In Middle English, the suffix lost its gender-specific function. Surnames that were originally agent nouns denoting occupations include Baxter (“baker”), Brewster (“beer maker”), and Webster (“weaver”); linguists are divided about whether these terms were exclusively applied to female practitioners of various crafts or referred to men as well. Another occupational term, teamster, originally referred to a wagon driver and later to a truck driver; this profession gave its name to the trade union called the International Brotherhood of Teamsters, which includes workers in many other trades.
Other words that we hardly notice include the suffix include gangster and mobster, both referring to members of criminal organizations. (Bankster is a recent play on the former word, alluding to allegedly felonious acts of bank executives.) A slang term for one who plays basketball is hoopster, and young and old people are referred to as youngsters and oldsters respectively.
Less common words include chorister (a member of a chorus), pollster (someone involved in developing or carrying out polls), and tipster (an informant). Most -ster constructions are neutral in tone, but two derisive appellations are hipster, referring to a person who self-consciously follows an alternative lifestyle, and scenester, someone who is ostentatiously immersed in a trendy social scene.
Various words for people who practice various forms of humor include gagster, jokester, prankster, punster, and quipster. Trickster refers to a dishonest person or to one who, like a stage magician, uses tricks to benignly deceive others; in folkloric studies, the term denotes a type of god or other supernatural figure who is cunning or who practices deception.
Two terms that refer to things rather than people are dragster (a car used in drag races) and roadster (a convertible sports car).
Although lobster is likely an agent noun—from loppe, an Old English word meaning “spider,” and -ster, its name is folk etymology based on the Latin word locustra (also the origin of locust)—some words that end in -ster are not related. Monster, for example, is based on the Latin term monstrum, meaning “monster” or “omen.”
When I was preparing to marry my British wife in 1981 I was intrigued to find her occupation listed as “Spinster” in her passport. At the time of issue she was in her mid-20s, hardly what would generally be considered a sprinsterish age.