-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.

Source: Webster's Revised Unabridged Dictionary (1913)