Old English -istre, from Proto-Germanic *-istrijon, feminine broker suffix made use of because the equivalent of masculine -ere (see -er (1)). Also used in center English to create nouns of action (meaning "an individual who ...") regardless of sex. The genderless broker noun usage apparently was a broader application associated with initial feminine suffix, from the north of England, but linguists disagree over whether this means that female domination of weaving and cooking positions, since represented in surnames such as Webster, Baxter, Brewster, an such like. (though spinster most likely holds an initially feminine ending). In addition whitester "one which bleaches cloth." In contemporary English, the suffix happens to be productive in forming derivative nouns (gamester, punster, etc.).
A suffix denoting the representative (originally a female), specially a one who does anything with skill or as a profession; as with spinster (originally, a female who spins), songster, baxter (= bakester), youngster.