Dictionary.net

A termination as in maltster, gamester, spinster, songster, denoting occupation. In the earliest times, and up to about the end of the thirteenth century, it was generally the sign of the feminine gender, corresponding to the masculine-ere or-er. In the fourteenth century it began to give place as a feminine termination to the Norman -ess. In modern literary English there is now only one feminine word with this suffix, viz. spinster, but huckster was used very late as a feminine; and in Scotch and Provincial English sewster is still used. "When the suffix -ster was felt no longer to mark the feminine distinctively, some new feminines were formed by the addition of the termination -ess to the -ster, as songstress and seamstress. "The suffix -ster now often marks the agent with more or less a sense of contempt and depreciation, as punster, trickster, gamester."-Dr. Morris. "But we cannot recognize the termination -ster as being, or as having been at some time past, a feminine formative in every instance. Not only does the present use of such old words as Baxter, huckster, maltster, songster, Webster, not to urge the more recent oldster, youngster roadster, make it hard to prove them all feminines; but even if we push our inquiries further back we nowhere find the group clearly defined as such except in modern Dutch. There was in Anglo-Saxon boecere and boecistre, and yet Pharaoh's baker in Genesis xl. is boecistre. Grimm conjectured that these nouns in -estre are all that is left of an older pair of declensions, whereof one was masculine in -estra, the other feminine in -estre."- J. Earle.