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.

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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Definitions of plain
1
plain (adj.)
clearly revealed to the mind or the senses or judgment;
in plain view
made his meaning plain
it is plain that he is no reactionary
plain (adj.)
not elaborate or elaborated; simple;
a plain blue suit
plain food
a plain rectangular brick building
stuck to the plain facts
plain (adj.)
lacking patterns especially in color;
Synonyms: unpatterned
plain (adj.)
not mixed with extraneous elements;
plain water
Synonyms: sheer / unmingled / unmixed
plain (adj.)
free from any effort to soften to disguise;
the plain and unvarnished truth
Synonyms: unvarnished
plain (adj.)
lacking embellishment or ornamentation;
a plain hair style
Synonyms: bare / spare / unembellished / unornamented
plain (adj.)
lacking in physical beauty or proportion;
a plain girl with a freckled face
Synonyms: homely
2
plain (n.)
extensive tract of level open land;
they emerged from the woods onto a vast open plain
Synonyms: field / champaign
plain (n.)
a basic knitting stitch made by putting the needle through the front of the stitch from the lefthand side;
Synonyms: knit / knit stitch / plain stitch
3
plain (v.)
express complaints, discontent, displeasure, or unhappiness;
Synonyms: complain / kick / sound off / quetch / kvetch
4
plain (adv.)
unmistakably (`plain' is often used informally for `plainly');
he is plain stubborn
Synonyms: obviously / evidently / manifestly / patently / apparently / plainly
From wordnet.princeton.edu