Etymology
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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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*plak- (2)

*plāk-, Proto-Indo-European root meaning "to strike."

It forms all or part of: apoplexy; cataplexy; complain; fling; paraplegia; plaint; plangent; plankton; planxty; plague; plectrum; quadriplegia.

It is the hypothetical source of/evidence for its existence is provided by: Greek plazein "to drive away," plēssein "to beat, strike;" Latin plangere "to strike, lament;" Old English flocan "to strike, beat;" Gothic flokan "to bewail;" German fluchen, Old Frisian floka "to curse."

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plangent (adj.)

"beating with a loud sound," 1822, from Latin plangentem (nominative plangens), present participle of plangere "to strike, beat" (from PIE root *plak- (2) "to strike"). Related: Plangently; plangency.

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plagio- 

before vowels plagi-, word-forming element meaning "slanting, oblique," from Greek plagios "oblique, slanting," from plagos "side," from PIE *plag- "flat, spread," variant form of root *plak- (1) "to be flat."

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quadriplegia (n.)

"paralysis of both arms and legs," 1895, a medical hybrid coined from Latin-based quadri- "four" + -plegia, as in paraplegia, which is ultimately from Greek plege "stroke," from root of plēssein "to strike" (from PIE root *plak- (2) "to strike"). A correct, all-Greek formation would be *tetraplegia.

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fluke (n.3)

"flatfish," Old English floc "flatfish," related to Old Norse floke "flatfish," flak "disk, floe," from Proto-Germanic *flok-, from PIE root *plak- (1) "to be flat." The parasite worm (1660s) so called from resemblance of shape.

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flag (n.2)

"flat stone for paving," c. 1600, ultimately from Old Norse flaga "stone slab," from Proto-Germanic *flago- (from extended form of PIE root *plak- (1) "to be flat"). Earlier in English as "piece cut from turf or sod" (mid-15c.), from Old Norse flag "spot where a piece of turf has been cut out," from flaga.

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planxty (n.)

in Irish music, "harp tune of a sportive and animated character" [OED], 1790, of unknown origin, evidently not a native Irish word. According to OED, some suggest ultimate derivation from Latin plangere "to strike, beat" (from PIE root *plak- (2) "to strike"). See also Katrin Thier, "Of Picts and Penguins -- Celtic Languages in the New Edition of the OED," in "The Celtic Languages in Contact," 2007.

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plankton (n.)

"organism that lives in a large body of water and is unable to swim against the current," 1891, from German Plankton (1887), coined by German physiologist Viktor Hensen (1835-1924) from Greek plankton, neuter of planktos "wandering, drifting," verbal adjective from plazesthai "to wander, drift," from plazein "to drive astray," from PIE root *plak- (2) "to strike." Related: Planktonic.

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paraplegia (n.)

"paralysis of the lower half of the body," 1650s, Latinized form of (Ionic) Greek paraplēgia "paralysis of one side of the body," from paraplēssein "strike at the side," paraplēssesthai "be stricken on one side," from para- "beside" (see para- (1)) + plēssein "to strike" (from PIE root *plak- (2) "to strike").

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