-ster Look up -ster at Dictionary.com
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 spinster probably carries an originally female ending). Also whitester "one who bleaches cloth." In Modern English, the suffix has been productive in forming derivative nouns (gamester, punster, etc.).