also *pletə-, Proto-Indo-European root meaning "to spread;" extension of root *pele- (2) "flat; to spread."
It forms all or part of: clan; flan; flat (adj.) "without curvature or projection;" flat (n.) "a story of a house;" flatter (v.); flounder (n.) "flatfish;" implant; piazza; place; plaice; plane; (n.4) type of tree; plant; plantain (n.2); plantar; plantation; plantigrade; plat; plate; plateau; platen; platform; platinum; platitude; Platonic; Plattdeutsch; platter; platypus; plaza; supplant; transplant.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit prathati "spreads out;" Hittite palhi "broad;" Greek platys "broad, flat;" Latin planta "sole of the foot;" Lithuanian platus "broad;" German Fladen "flat cake;" Old Norse flatr "flat;" Old English flet "floor, dwelling;" Old Irish lethan "broad."
c. 1600, Cornish name for a type of fish (also known as horse mackerel) abundant on the British coast; a name of uncertain origin, perhaps a variant of shad. OED compares Welsh ysgaden "herrings," Norwegian dialectal skad, Swedish skädde "flounder."
In July, 1834, as Mr. Yarrell informs us, most extraordinary shoals passed up the channel along the coast of Glamorganshire; their passage occupied a week, and they were evidently in pursuit of the fry of the herring. The water appeared one dark mass of fish, and they were caught by cart-loads, and might even be baled out of the water by the hands alone. ["British Fish and Fisheries," 1849]