Etymology
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plain (adj.)

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.

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plain (n.)

"level country, expanse of level or nearly level ground," c. 1300 (in reference to Salisbury Plain), from Old French plain "open countryside," from Latin planum "level ground, plain," noun use of neuter of planus (adj.) "flat, even, level" (from PIE root *pele- (2) "flat; to spread"). Latin planum was used for "level ground" but much more common was campus.

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plain Jane 

"homely or unattractive woman, girl without beauty," attested by 1912, a rhyming formation from plain (adj.).

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plain-spoken (adj.)

"speaking or spoken with unreserved sincerity," 1670s, from plain (adj.) + -spoken.

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plain-song (n.)

also plainsong, unisonous vocal music used in the Christian churches in the earliest centuries, mid-15c., translating Latin cantus planus, French plain chant; see plain (adj.) + song (n.).

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plainly (adv.)

late 14c., "frankly, candidly; without a doubt, truly; in a clear and distinct manner; simply, straightforwardly," from plain (adj.) + -ly (2). Meaning "simply, frugally" is from 1560s.

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plainness (n.)

c. 1300, plainnes, "flatness, level ground, flat surface;" late 14c., "smoothness, evenness," from plain (adj.) + -ness. Meaning "clarity, lucidity" is from mid-15c.; that of "open conduct" is from 1540s; that of "absence of ornament" is from 1580s.

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plain clothes (n.)

"ordinary dress of civil life" (as opposed to military uniform), 1822; in reference to police detectives, it is attested from 1842. Also plainclothes.

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Plains (n.)

"lands of the American Midwest lying from roughly to the 104th meridian to the eastern slopes of the Rockies," 1755 (in singular form from 1680s), see plain (n.). Plains Indian is attested from 1844.

This region has a gradual slope from the mountains to the Missouri and Mississippi rivers, but is nowhere broken by any conspicuous ranges of hills. It is a region of small precipitation, wooded only along the banks of the streams, and not always there. The Plains and the prairies are not properly the same, from either a geographical or a climatological point of view. [Century Dictionary]
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