Etymology
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platypus (n.)

"Australian duck-mole," 1799, from Modern Latin, from Greek platypous, literally "flat-footed," from platys "broad, flat" (from PIE root *plat- "to spread") + pous "foot," from PIE root *ped- "foot." Originally the genus name, but entomologists had given it earlier to a genus of beetles; it was retained for the species after the genus name was changed in 1800 to Ornithorhyncus. OED has Australian platypussary (1945) "enclosure in which platypuses are kept."

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octopod 

1826 (adj.), "eight-footed or eight-armed;" 1835 (n.) "an eight-footed or eight-armed animal," especially an octopus, from Latinized form of Greek oktōpod-, stem of oktōpous (see octopus).

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tetrapod (n.)
"four-footed animal, quadruped," 1826, from Modern Latin tetrapodus, from Greek tetrapous "four-footed," as a noun, "four-footed animal," from tetra- "four" (from PIE root *kwetwer- "four") + pous "foot" (from PIE root *ped- "foot").
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quadruped (n.)

"a four-footed animal," especially "a four-footed mammal," 1640s, from French quadrupède (16c.), from Latin quadrupes (genitive quadrupedis) "four-footed, on all fours," also, as a noun, "a four-footed animal," from quadri- "four" (from PIE root *kwetwer- "four") + pes "foot" (from PIE root *ped- "foot").

The adjective is attested from 1741, "four-footed, having four limbs fitted for sustaining the body and locomotion, habitually going on all fours." Related: Quadrupedal (1610s). In zoology, quadrumane (from Latin manus "hand") was "a four-handed animal," in reference to monkeys, apes, lemurs, etc.; attested by 1786.

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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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club-foot (n.)

also clubfoot, "deformed foot," 1530s, from club (n.) + foot (n.). Related: Club-footed.

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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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plani- 

word-forming element meaning "level, flat, plane," from Latin plani-, from planus "flat, level" (from PIE root *pele- (2) "flat; to spread").

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pinniped (n.)

"a fin-footed mammal," one having feet like fins or flippers, especially of the group of fin-footed aquatic carnivorous quadruped mammals that includes seals, sea-lions, and walruses, 1842, from Modern Latin Pinnipedia, suborder of aquatic carnivorous mammals (seals and walruses), literally "having feet as fins," from Latin pinna in its secondary sense "fin" (see pin (n.)) + pes, genitive pedis "foot" (from PIE root *ped- "foot").

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flatten (v.)
late 14c., "to prostrate oneself," also "to fall flat," from flat (adj.) + -en (1). Transitive meaning "to make flat" is 1620s. Related: Flattened; flattening.
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