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.
Old English sang "voice, song, art of singing; metrical composition adapted for singing, psalm, poem," from Proto-Germanic *songwho- (source also of Old Norse söngr, Norwegian song, Swedish sång, Old Saxon, Danish, Old Frisian, Old High German, German sang, Middle Dutch sanc, Dutch zang, Gothic saggws), from PIE *songwh-o- "singing, song," from *sengwh- "to sing, make an incantation" (see sing (v.)).
Phrase for a song "for a trifle, for little or nothing" is from "All's Well" III.ii.9 (the identical image, por du son, is in Old French. With a song in (one's) heart "feeling joy" is first attested 1930 in Lorenz Hart's lyric. Song and dance as a form of vaudeville act is attested from 1872; figurative sense of "rigmarole" is from 1895.