-ster

English agent noun ending, corresponding to Latin -or. In native words it represents Old English -ere (Old Northumbrian also -are) "man who has to do with," from Proto-Germanic *-ari (cognates: German -er, Swedish -are, Danish -ere), from Proto-Germanic *-arjoz. Some believe this root is identical with, and perhaps a borrowing of, Latin -arius (see -ary).

Generally used with native Germanic words. In words of Latin origin, verbs derived from past participle stems of Latin ones (including most verbs in -ate) usually take the Latin ending -or, as do Latin verbs that passed through French (such as governor); but there are many exceptions (eraser, laborer, promoter, deserter; sailor, bachelor), some of which were conformed from Latin to English in late Middle English.

The use of -or and -ee in legal language (such as lessor/lessee) to distinguish actors and recipients of action has given the -or ending a tinge of professionalism, and this makes it useful in doubling words that have a professional and a non-professional sense (such as advisor/adviser, conductor/conducter, incubator/incubater, elevator/elevater).

mid-14c., "female spinner of thread," from Middle English spinnen "spin fibers into thread" (see spin (v.)) + -stere, feminine suffix (see -ster). Unmarried women were supposed to occupy themselves with spinning, hence the word came to be "the legal designation in England of all unmarried women from a viscount's daughter downward" [Century Dictionary] in documents from 1600s to early 1900s, and by 1719 the word was being used generically for "woman still unmarried and beyond the usual age for it."

Spinster, a terme, or an addition in our Common Law, onely added in Obligations, Euidences, and Writings, vnto maids vnmarried. [John Minsheu, "Ductor in Linguas," 1617]

Strictly in reference to those who spin, spinster also was used of both sexes (compare webster, Baxter, brewster) and so a double-feminine form emerged, spinstress "a female spinner" (1640s), which by 1716 also was being used for "a maiden lady." Related: Spinsterhood.