Etymology
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flatfish (n.)
also flat-fish, 1710, from flat (adj.) + fish (n.). So called from the shape.
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Planaria (n.)

flat worm-like animal, 1819, from Modern Latin (1776) noun use of fem. of Late Latin planarius, literally "on level ground" (here used to mean "flat"), from Latin planum, planus "flat, level, even, plain" (from PIE root *pele- (2) "flat; to spread"). Related: Planarian.

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flatness (n.)
mid-15c., "state or quality of being flat," from flat (adj.) + -ness.
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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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trivet (n.)
three-legged iron stand, 12c., trefet, probably from a noun use of Latin tripedem (nominative tripes) "three-footed," from tri- "three" (see three) + pes "foot" (from PIE root *ped- "foot").
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placenta (n.)

1670s of plants, "part of the ovary of flowering plants which bears the ovules," 1690s of mammals, "organ of attachment of a vertebrate embryo or fetus to the wall of the uterus or womb of the female," from Modern Latin placenta uterina "uterine cake" (so called 16c. by Italian anatomist Realdo Colombo), from Latin placenta "a cake, flat cake," from Greek plakoenta, accusative of plakoeis "flat," from plax "flat, flat land, surface, plate," from PIE root *plak- (1) "to be flat." So called from the shape.

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pancake (v.)
"to squeeze flat," 1879, from pancake (n.). Later, of aircraft, "to fall flat" (1911), with figurative extension. Related: Pancaked; pancaking.
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planar (adj.)

"lying in or otherwise related to a plane, flat," 1850, from Latin planaris "level, flat," from planum "plane" (from PIE root *pele- (2) "flat; to spread"). An earlier word in the same sense was planary (1660s).

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decapod (n.)

1819, "ten-legged animal, type of crustacean having ten legs" (crabs, lobsters, shrimp), from French décapode (1806), from Modern Latin Decapoda (animalia), from Greek dekapoda, neuter plural of dekapous "ten-footed" (see ten + foot (n.)). From 1885 in reference to a type of locomotive with ten driving-wheels.

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plat (n.)

"flat piece of ground," mid-15c. (mid-13c. in surnames), a variant of plot (n.) assimilated to Middle English plat (adj.) "flat," which is from Old French plat "flat, stretched out" (see plateau (n.)). See OED plat sb.3 for full explanation.

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