The Collaborative International Dictionary of English v.0.48:

-ster \-ster\ [OE. & AS. -estre, -istre.]
   A suffix denoting the agent (originally a woman), especially
   a person who does something with skill or as an occupation;
   as in spinster (originally, a woman who spins), songster,
   baxter (= bakester), youngster.
   [1913 Webster]

   Note: Brewing, baking, and weaving were formerly feminine
         labors, and consequently brewster, baxter, and webster
         meant, originally, the woman (not the man) who brews,
         bakes, or weaves. When men began to perform these
         duties the feminine appellations were retained.
         [1913 Webster]