Etymology
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plank (n.)

late 13c. (c. 1200 as a surname), "thick board used in construction," from Old North French planke, a variant of Old French planche "plank, slab, little wooden bridge" (12c.), from Late Latin planca "broad slab, board," probably from Latin plancus "flat, flat-footed," from a nasalized variant of PIE root *plak- (1) "to be flat." Planche itself was also used in Middle English.

Technically, timber sawed to measure 2 to 6 inches thick, 9 inches or more wide, and 8 feet or more long. The political sense of "article or paragraph formulating a distinct principle in a party platform" is U.S. coinage from 1848, based on the double sense of platform. To be made to walk the plank, "be forced to walk along a plank laid across the bulwarks of a ship until one reaches the end and falls into the sea," popularly supposed to have been a pirate form of execution, is attested from 1789, and most early references are to slave-ships disposing of excess human cargo in crossing the ocean.

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plank (v.)

"to cover or lay with planks," early 15c., from plank (n.). Related: Planked; planking.

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gang-plank (n.)
also gangplank, 1842, American English, from gang in its nautical sense of "a path for walking, passage" (see gangway) + plank. Replacing earlier gang-board.
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planchet (n.)

"metal disk out of which a coin is made," 1610s, from French planchette, literally "a small board," a diminutive of Old French planche (12c.; the source of plank), from Late Latin planca "board, slab, plank," which is probably from Latin plancus "flat, flat-footed," from a nasalized variant of the PIE root *plak- (1) "to be flat."

The small, heart-shaped planchette on its three legs, used in automatism and on Ouija boards, is a re-borrowing of the French word, by 1860.

If the tips of the fingers of one person, or of two, are placed lightly upon it, the board will often, after a time, move without conscious effort on the part of the operator, and the pencil-point will, it is said, trace lines, words, and even sentences. It was invented about 1855, and was for a time an object of not a little superstition. [Century Dictionary]
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*plak- (1)

also *plāk-, Proto-Indo-European root meaning "to be flat;" extension of root *pele- (2) "flat; to spread."

It forms all or part of: flag (n.2) "flat stone for paving;" flagstone; flake (n.) "thin flat piece,; flaw; floe; fluke (n.3) "flatfish;" placenta; plagal; plagiarism; plagio-; planchet; plank.

It is the hypothetical source of/evidence for its existence is provided by: Greek plakoeis "flat," plax "level surface, anything flat;" Lettish plakt "to become flat;" Old Norse flaga "layer of earth," Norwegian flag "open sea," Old English floh "piece of stone, fragment," Old High German fluoh "cliff."

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board (n.1)

"piece of timber sawn flat and thin, longer than it is wide, wider than it is thick, narrower than a plank;" Old English bord "a plank, flat surface," from Proto-Germanic *burdam (source also of Old Norse borð "plank," Dutch bord "board," Gothic fotu-baurd "foot-stool," German Brett "plank"), perhaps from a PIE verb meaning "to cut." See also board (n.2), with which this is so confused as practically to form one word (if indeed they were not the same word all along).

In late Old English or early Middle English the sense was extended to include "table;" hence the transferred meaning "food" (early 14c.), as "that which is served upon a table," especially "daily meals provided at a place of lodging" (late 14c.). Compare boarder, boarding, and Old Norse borð, which also had a secondary sense of "table" and an extended sense "maintenance at table." Hence also above board "honest, open" (1610s; compare modern under the table "dishonest"). A further extension is to "table where council is held" (1570s), then transferred to "leadership council, persons having the management of some public or private concern" (1610s), as in board of directors (1712).

"Bow to the board," said Bumble. Oliver brushed away two or three tears that were lingering in his eyes; and seeing no board but the table, fortunately bowed to that.

Meaning "table upon which public notices are written" is from mid-14c. Meaning "table upon which a game is played" is from late 14c. Meaning "thick, stiff paper" is from 1530s. Boards "stage of a theater" is from 1768.

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knot-hole (n.)
also knothole, "hole left in a plank or board after a knot has dropped out," 1726, see knot (n.) + hole (n.).
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gunwale (n.)
"uppermost edge of a ship's side," mid-15c., gonne walle, from gun (n.) + wale "plank" (see wale). Originally a platform on the deck of a ship to support the mounted guns.
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see-saw (n.)

also seesaw, 1630s, in see-saw-sacke a downe (like a Sawyer), words in a rhythmic jingle used by children and repetitive-motion workers, probably imitative of the rhythmic back-and-forth motion of sawyers working a two-man pit saw (see saw (n.1). Ha ha.).

In reference to a children's sport of going alternately up and down on a plank balanced on some support, it is recorded from 1704; the figurative sense of this is from 1714. Applied from 1824 to the plank arranged and adjusted for the game. Also compare teeter-totter under teeter (v.). 

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deal (n.2)

"plank or board," especially of fir or pine, late 14c., dele, from Low German (compare Middle Low German dele), from Proto-Germanic *theljon." From late 13c. in surnames. An Old English derivative was þelu"hewn wood, board, flooring."

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