Old English smoð "smooth, serene, calm," variant of smeðe "free from roughness, not harsh, polished; soft; suave; agreeable," of unknown origin and with no known cognates. Of words, looks, "pleasant, polite, sincere" late 14c., but later "flattering, insinuating" (mid-15c.). Slang meaning "superior, classy, clever" is attested from 1893. Sense of "stylish" is from 1922.
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, such as Smithfield, Smedley.
late Old English smoþ "to make smooth," replacing smeðan "to smooth, soften, polish; appease, soothe;" smeðian "smoothen, become smooth," from the source of smooth (adj.). Meaning "to make smooth" is c. 1200. Related: Smoothed; smoothing. Middle English also had a verb form smoothen (mid-14c.).