c.1300, "flat, smooth," from Old French plain "flat, smooth, even" (12c.), from Latin planus "flat, even, level" (see plane (n.1)). Sense of "evident" is from, c.1300; that of "free from obstruction" is early 14c.; meaning "simple, sincere, ordinary" is recorded from late 14c., especially of dress, "unembellished, without decoration."
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. As an adverb from early 14c. Plain English is from c.1500. Plain dealer "one who deals plainly or speaks candidly" is from 1570s, marked "Now rare" in OED 2nd edition. To be as plain as the nose on (one's) face is from 1690s.
"level country," 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" (see plane (n.1)). Latin planum was used for "level ground" but much more common was campus.
Programming LAnguage for INteraction. Pascal-like, with extensions for database, string handling, exceptions and pattern matching. "Revised Report on the Programming Language PLAIN", A. Wasserman, SIGPLAN Notices 6(5):59-80 (May 1981).
(1.) Heb. 'abel (Judg. 11:33), a "grassy plain" or "meadow." Instead of "plains of the vineyards," as in the Authorized Version, the Revised Version has "Abel-cheramim" (q.v.), comp. Judg. 11:22; 2 Chr. 16:4. (2.) Heb. 'elon (Gen. 12:6; 13:18; 14:13; 18:1; Deut. 11:30; Judg. 9:6), more correctly "oak," as in the Revised Version; margin, "terebinth." (3.) Heb. bik'ah (Gen. 11:2; Neh. 6:2; Ezek. 3:23; Dan. 3:1), properly a valley, as rendered in Isa. 40:4, a broad plain between mountains. In Amos 1:5 the margin of Authorized Version has "Bikathaven." (4.) Heb. kikar, "the circle," used only of the Ghor, or the low ground along the Jordan (Gen. 13:10-12; 19:17, 25, 28, 29; Deut. 34:3; 2 Sam. 18:23; 1 Kings 7:46; 2 Chr. 4:17; Neh. 3:22; 12:28), the floor of the valley through which it flows. This name is applied to the Jordan valley as far north as Succoth. (5.) Heb. mishor, "level ground," smooth, grassy table-land (Deut. 3:10; 4:43; Josh. 13:9, 16, 17, 21; 20:8; Jer. 48:21), an expanse of rolling downs without rock or stone. In these passages, with the article prefixed, it denotes the plain in the tribe of Reuben. In 2 Chr. 26:10 the plain of Judah is meant. Jerusalem is called "the rock of the plain" in Jer. 21:13, because the hills on which it is built rise high above the plain. (6.) Heb. 'arabah, the valley from the Sea of Galilee southward to the Dead Sea (the "sea of the plain," 2 Kings 14:25; Deut. 1:1; 2:8), a distance of about 70 miles. It is called by the modern Arabs the Ghor. This Hebrew name is found in Authorized Version (Josh. 18:18), and is uniformly used in the Revised Version. Down through the centre of this plain is a ravine, from 200 to 300 yards wide, and from 50 to 100 feet deep, through which the Jordan flows in a winding course. This ravine is called the "lower plain." The name Arabah is also applied to the whole Jordan valley from Mount Hermon to the eastern branch of the Red Sea, a distance of about 200 miles, as well as to that portion of the valley which stretches from the Sea of Galilee to the same branch of the Red Sea, i.e., to the Gulf of Akabah about 100 miles in all. (7.) Heb. shephelah, "low ground," "low hill-land," rendered "vale" or "valley" in Authorized Version (Josh. 9:1; 10:40; 11:2; 12:8; Judg. 1:9; 1 Kings 10:27). In Authorized Version (1 Chr. 27:28; 2 Chr. 26:10) it is also rendered "low country." In Jer. 17:26, Obad. 1:19, Zech. 7:7, "plain." The Revised Version renders it uniformly "low land." When it is preceded by the article, as in Deut. 1:7, Josh. 11:16; 15:33, Jer. 32:44; 33:13, Zech. 7:7, "the shephelah," it denotes the plain along the Mediterranean from Joppa to Gaza, "the plain of the Philistines." (See VALLEY.)