Entries linking to plainly
c. 1300, "flat, smooth," from Old French plain "flat, smooth, even" (12c.), from Latin planus "flat, even, level" (from PIE root *pele- (2) "flat; to spread"). Sense of "explicit, clear, evident" is from late 14c.; that of "free from obstruction" is mid-14c.; meaning "simple, sincere, ordinary" is recorded from late 14c., especially of dress, "unembellished, without decoration, unadorned." Of words, speech, etc., "direct and to the point," late 14c. As an adverb from late 14c.
In reference to the dress and speech of Quakers, it is recorded from 1824; of Amish and Mennonites, from 1894 (in the Dutch regions of Pennsylvania Plain with the capital is shorthand adjective for "Amish and Old Order Mennonite"). Of appearance, as a euphemism for "ill-favored, ugly" it dates from 1749. Of envelopes from 1913.
Plain English is from c. 1500. Plain dealer "one who speaks his opinions candidly; one who is frank, honest, and open" is from 1570s, marked "Now rare" in OED 2nd edition, though it survives since 1842 as the name of the main newspaper of Cleveland, Ohio. To be as plain as the nose on (one's) face "obvious" is from 1690s.
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.