E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
says E. Adams (English Language,) properly means a bar-maid; -ster is the Anglo-Saxon feminine suffix -estre, which remains in spin-ster (a female spinner).
This is only a half-truth. After the thirteenth century, the suffix -ster was used for an agent of either sex. We have barrister, gamester, punster, etc., and Wickliffe uses songster for a male singer (See Dr. Morris: Historic Outlines, p. 89.)