smooth adj.

"having a uniform surface, not rough," a Middle English form, from Old English smoð, a variant of smeðe "free from roughness, not harsh, polished; soft; suave; agreeable," which is of unknown origin and has no known cognates.

The usual form in early Middle English was smeeth, but after c. 1400 this was archaic or provincial, however it is preserved in places names, such as Smithfield, Smedley. For the variation, compare Middle English smeken "to smoke, give off smoke," alongside smoken (v.).

Of words, looks, "pleasant, polite, sincere" late 14c., but later "flattering, insinuating" (mid-15c.). The meaning "without shocks or jolts" is from 1756. The slang meaning "superior, classy, clever" is attested from 1893; the sense of "stylish" is from 1922.

Smooth-bore ("not rifled") 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."

"make smooth," Middle English smothen, a form attested from late Old English (smoþ), from the source of smooth (adj.). From 14c., smooth replaced earlier smeeth, from Old English smeðan "to smooth, soften, polish; appease, soothe;" smeðian "smoothen, become smooth." Smeeth survived after Middle English only as an archaic or provincial word.

The meaning "make smooth" is c. 1200. The figurative sense of "make agreeable, soften" is by 1590s. Of a graph, etc., "modify values to remove irregularities," by 1889. Related: Smoothed; smoothing. Alternative verb smoothen is from 1630s; OED reports it "in frequent use c 1820-30, esp. by Landor."