late 14c., smotheli, "in a smooth manner, blandly," from smooth (adj.) + -ly (2). Earlier was smetheli (c. 1200). The meaning "without impediment or complications" is from 1660s.
Entries linking to smoothly
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."
-ly 2
common adverbial suffix, forming from adjectives adverbs signifying "in a manner denoted by" the adjective, Middle English, from Old English -lice, from Proto-Germanic *-liko- (cognates: Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko); see -ly (1). Cognate with lich, and identical with like (adj.).
Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.