-ster

CIDE DICTIONARY

-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]
" 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]

OXFORD DICTIONARY

-ster, suffix denoting a person engaged in or associated with a particular activity or thing (brewster; gangster; youngster).

Etymology

OE -estre etc. f. Gmc