Old English smoð "free from roughness, not harsh," of unknown origin. Sense of "pleasant, polite, sincere" first recorded late 14c. Slang meaning "superior, classy, clever" is attested from 1893. Smooth-bore in reference to guns is from 1812. smooth talk (v.) is recorded from 1950. A 1599 dictionary has smoothboots "a flatterer, a faire spoken man, a cunning tongued fellow." The usual Old English form was smeðe, and there is a dialectal smeeth found in places names, e.g. Smithfield, Smedley.
late Old English smeþan, smeþian "to become smooth," from the source of smooth (adj.). Meaning "to make smooth" is c.1200. Related: Smoothed; smoothing.