Etymology
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brill (n.)
kind of flat fish, late 15c., of unknown origin.
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pontoon (n.)

"flat-bottomed boat" (especially, in military engineering, one to support a temporary bridge over a river), 1670s, from French pontoon, from Old French ponton (14c.) "bridge, drawbridge, boat-bridge; flat-bottomed boat," from Latin pontonem (nominative ponto) "flat-bottomed boat," from pons "bridge" (see pons). Pontoon bridge "roadway supported on pontoons" is recorded by 1778. Related: Pontooneer (n.).

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prostrate (v.)

early 15c., prostraten, "prostrate oneself, fall down flat, bow with the face to the ground" (in humility or submission), from prostrate (adj.). Transitive sense of "throw down, lay flat, overthrow" is by 1560s. Related: Prostrated; prostrating.

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Plattdeutsch 

"Low German dialect of northern Germany," 1814, from German, from Dutch platduits, literally "flat (or low) German," from plat "flat, plain, clear" + duits "German" (see Dutch). In contrast to the speech of the upland parts of Germany (High German).

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vapid (adj.)
1650s, "flat, insipid" (of drinks), from Latin vapidus "flat, insipid," literally "that has exhaled its vapor," related to vappa "stale wine," and probably to vapor "vapor." Applied from 1758 to talk and writing deemed dull and lifeless. Related: Vapidly; vapidness.
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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.

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cacoon (n.)
large, flat bean from an African shrub, 1846, from some African word.
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plane (n.3)

"tool for smoothing surfaces," mid-14c., from Old French plane, earlier plaine (14c.) and directly from Late Latin plana, back-formation from planare "make level," from Latin planus "level, flat, smooth" (from PIE root *pele- (2) "flat; to spread").

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plane (v.2)

"soar, glide on motionless wings," early 15c., planen, from Old French planer "to hover (as a bird), to lie flat," from plan (n.) "plane," or perhaps via Medieval Latin; in either case from Latin planum "flat surface" (from PIE root *pele- (2) "flat; to spread"), on notion of bird gliding with flattened wings. Of boats, etc., "to skim over the surface of water," it is attested by 1913. Related: Planed; planing.

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pianissimo (adv.)

musical instruction, "with the minimum of force or loudness," 1724, from Italian pianissimo "very softly," superlative of piano, which is ultimately is from Latin planus "flat, smooth, even," later "soft" (from PIE root *pele- (2) "flat; to spread").

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