I. plāk-1
Also plak-. To be flat. Earliest form *pleə₂k-, colored to *plaə₂k-, contracted to *plāk-. Extension of pelə-2.
Derivatives include fluke1, flaw1, plead, placenta, and archipelago.
1. floe, from Old Norse flō, layer, coating, from Germanic *flōhō.
2. Variant form *plāg-.
a. fluke1, from Old English flōc, flatfish, from Germanic *flōk-;
b. flake1, from Middle English flake, flake, from a Scandinavian source probably akin to Norwegian flak, flat piece, flake, from Germanic *flakaz;
c. flake2, from Old Norse flaki, fleki, hurdle, from Germanic *flak-.
3. Extended form *plakā-. flag4, flaw1, from Old Norse flaga, layer of stone, from Germanic *flagō.
4. Possibly suffixed (stative) form *plak-ē-, to be calm (as of the flat sea). placebo, placid, plea, plead, pleasant, please; complacent, from Latin placēre, to please, be agreeable.
5. Root noun *plak-. supplicate, supple, from Latin supplex, suppliant (whence denominative supplicāre, to beg humbly, first attested in Archaic Latin as sub vos placō, I entreat you; sub, under; see upo.)
6. Lengthened suffixed form *plāk-ā-. placable, placate, from Latin plācāre, to calm (causative of placēre).
7. Nasalized form *pla-n-k-. planchet, plank, from Latin plancus, flat, flat-footed.
8. Variant form *plag-.
a. Perhaps Latin plaga, net (? < “something extended”): plagiary;
b. plagal, plagio-, playa, from Greek plagos, side.
9. Root form *plak-. placenta, placoid; leukoplakia, placoderm, from Greek plax, flat, flat land, surface, plate.
10. Possible variant form *pelag-. pelagic; archipelago, from Greek pelagos, sea.
[Pokorny 1. plā-k- 831.]
  II. plāk-2
To strike. Oldest form *pleə₂k-, colored to *plaə₂k-, contracted to *plāk-.
1. Nasalized variant forms *pla-n-k-, *pla-n-g-.
a. fling, from Middle English flingen, to fling, from a Scandinavian source akin to Old Norse flengja, to flog, whip, from Germanic *flang-;
b. plaint, plangent; complain, from Latin plangere, to strike (one's own breast), lament;
c. suffixed form *plang-yo-. plankton, from Greek plazein, to drive away, turn aside.
2. Variant form *plāg-. plague, from Latin plāga, a blow, stroke.
3. Suffixed form *plāk-yo-. plectrum, -plegia, plexor; apoplexy, cataplexy, paraplegia, from Greek plēssein, to beat, strike.
[Pokorny 2. plāk- 832.]

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Universalium. 2010.