Etymology
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Words related to plain

*pele- (2)
*pelə-, Proto-Indo-European root meaning "flat; to spread."

It forms all or part of: airplane; dysplasia; ectoplasm; effleurage; esplanade; explain; explanation; feldspar; field; flaneur; floor; llano; palm (n.1) "flat of the hand;" palm (n.2) "tropical tree;" palmy; piano; pianoforte; plain; plan; planar; Planaria; plane (n.1) "flat surface;" plane (n.3) "tool for smoothing surfaces;" plane (v.2) "soar, glide on motionless wings;" planet; plani-; planisphere; plano-; -plasia; plasma; plasmid; plasm; -plasm; -plast; plaster; plastic; plastid; -plasty; Polack; Poland; Pole; polka; protoplasm; veldt.

It is the hypothetical source of/evidence for its existence is provided by: Greek plassein "to mold," plasma "something molded or created;" Latin planus "flat, level, even, plain, clear;" Lithuanian plonas "thin;" Celtic *lanon "plain;" Old Church Slavonic polje "flat land, field," Russian polyi "open;" Old English feld, Middle Dutch veld "field."
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plain Jane 

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

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.

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.

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.).

plain-spoken (adj.)

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

plan (n.)

1670s as a technical term in perspective drawing; more generally by 1706 as "the representation of anything drawn on a plane; a drawing, sketch, or diagram of any object," from French plan "ground plot of a building, map," literally "plane surface" (mid-16c.), from Latin planum "level or flat surface," noun use of adjective planus "level, flat" (from PIE root *pele- (2) "flat; to spread").

The notion is of "a drawing on a flat surface." A doublet of plain via a later, learned French form. The meaning "scheme of action, formulated scheme for the accomplishment of some object or attainment of an end" is by 1713.

flood-plain (n.)
1844, from flood (n.) + plain (n.).
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]