Source : Webster's Revised Unabridged Dictionary (1913)

-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.

   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.