Online Etymology Dictionary

Plasticine: modeling clay substitute, 1897, from plastic (adj.) + -ine (2).
plasticity: 1782, from plastic + -ity.
plastid (n.): 1876, from German plastid, coined by Haeckel from Greek plastos "molded, formed" (see plaster) + -id.
plastron (n.): c. 1500, from Middle French plastron "breastplate," from Italian piastrone, augmentative of piastra "breastplate, thin metal plate" (see piaster). As an animal part, from 1813; as an article of dress, 1876.
plat (n.): "piece of ground," 1510s, from plot (q.v.), assimilated to Middle English adjective plat "flat," which is from Old French plat "flat, stretched out" (see plateau (n.)). See OED for full explanation.
plat (v.): "to interweave," late 14c., variant of plait (v.). Related: Platted; platting.
plate (n.): mid-13c., "flat sheet of gold or silver," also "flat, round coin," from Old French plate "thin piece of metal" (late 12c.), from Medieval Latin plata "plate, piece of metal," perhaps via Vulgar Latin *plattus, formed on model of Greek platys "flat, broad" (see plaice (n.)). The cognate in Spanish (plata) and Portuguese (prata) has become the usual word for "silver," superseding argento via shortening of *plata d'argento "plate of silver, coin." Meaning "table utensils" (originally of silver or gold only) is from Middle English. Meaning "shallow dish for food," now usually of china or earthenware, originally of metal or wood, is from mid-15c. Baseball sense is from 1857. Geological sense is first attested 1904; plate tectonics first recorded 1969. Plate-glass first recorded 1727.
plate (v.): late 14c., from plate (n.). Related: Plated; plating.
plateau (n.): 1796, "elevated tract of relatively level land," from French plateau "table-land," from Old French platel (12c.) "flat piece of metal, wood, etc.," diminutive of plat "flat surface or thing," noun use of adjective plat "flat, stretched out" (12c.), perhaps from Vulgar Latin *plattus, from Greek platys "flat, wide, broad" (see plaice). Meaning "stage at which no progress is apparent" is attested from 1897, originally in psychology of learning. In reference to sexual stimulation from 1960.
plateau (v.): 1952, from plateau (n.). Related: Plateaued; plateauing.
platelet (n.): 1895, formed in English from plate (n.) + diminutive suffix -let.
platen (n.): 1540s, from Middle French plateine, from Old French platine "flat piece, metal plate" (13c.), perhaps altered (by influence of plat "flat") from patene, from Latin patena "pan."
platform (n.): 1540s, "plan of action, scheme, design," from Middle French plateforme, platte fourme, literally "flat form," from Old French plat "flat" (see plateau (n.)) + forme "form" (see form (n.)). The literal sense of "raised, level surface" in English is first recorded 1550s. Political meaning, "statement of party policies," is from 1803, probably originally an image of a literal platform on which politicians gather, stand, and make their appeals, perhaps influenced by earlier sense of "set of rules governing church doctrine" (first attested 1570s). Railroad station sense is from 1838.
platinum (n.): metallic element, 1812, Modern Latin, from Spanish platina "platinum," diminutive of plata "silver," from Old French plate or Old Provençal plata "sheet of metal" (see plate (n.)). The metal looks like silver, and the Spaniards at first thought it an inferior sort of silver, hence the name platina. It was first obtained from Spanish colonies in Mexico and Colombia, brought to Europe in 1735, and identified as an element 1741. Taken into English as platina (c. 1750), it took its modern form (with element ending -ium) in 1812, at the time the names of elements were being regularized. As a shade of blond hair, attested from 1931. As a designation for a recording that has sold at least one million copies, it is attested from 1971.
platitude (n.): 1812, "dullness," from French platitude "flatness, vapidness" (late 17c.), from Old French plat "flat" (see plateau (n.)); formed on analogy of latitude, etc. Meaning "a flat, dull, or commonplace remark" is recorded from 1815. Related: Platitudinous. Hence platitudinarian (n.), 1855; platitudinize (1867).
Platonic (adj.): 1530s, "of or pertaining to Greek philosopher Plato" (429 B.C.E.-c. 347 B.C.E.), from Latin Platonicus, from Greek Platonikos. The name is Greek Platon, properly "broad-shouldered" (from platys "broad;" see plaice (n.)). His original name was Aristocles. The meaning "love free of sensual desire" (1630s), which the word usually carries nowadays, is a Renaissance notion; it is based on Plato's writings in "Symposium" about the kind of interest Socrates took in young men, which originally had no reference to women. Related: Platonically.
Platonism (n.): 1560s, from Plato (see Platonic) + -ism.
Platonist (n.): 1540s, from Plato (see Platonic) + -ist.
platoon (n.): 1630s, from French peloton "platoon, group of people," from Middle French peloton (15c.), literally "little ball," hence, "agglomeration," diminutive of Old French pelote "ball" (see pellet).
platoon (v.): in baseball, "to alternate (a player) with another in the same position," 1967, from platoon (n.), which had been used in team sports since 1941.
Plattdeutsch: "Low German dialect of northern Germany," 1814, from German, from Dutch platduits, literally "flat (or low) German," from plat "flat, plain, clear" + duits "German" (see Dutch). In contrast to the speech of the upland parts of Germany.
platter (n.): late 13c., platere, from Anglo-French plater, from Old French plate "metal plate" (see plate (n.)).
platypus (n.): Australian duck-mole, 1799, from Modern Latin, from Greek platypous, literally "flat-footed," from platys "broad, flat" (see plaice (n.)) + pous "foot," from PIE root *ped- (1) "a foot" (see foot (n.)).
Orig. the generic name, but, having already been given to a genus of beetles, it was in 1800 changed for Ornithorhyncus. [OED]
plaudit (n.): 1620s, short for plaudite "an actor's request for applause" (1560s), from Latin plaudite! "applaud!" second person plural imperative of plaudere "to clap, strike, beat; applaud, approve," of unknown origin (also in applaud, explode). This was the customary appeal for applause that Roman actors made at the end of a play. In English, the -e went silent then was dropped.
plausibility (n.): 1590s, from plausible + -ity.
plausible (adj.): 1540s, "acceptable, agreeable," from Latin plausibilis "deserving applause, acceptable," from plaus-, past participle stem of plaudere "to applaud" (see plaudit). Meaning "having the appearance of truth" is recorded from 1560s. Related: Plausibly.
play (v.): Old English plegan, plegian "move rapidly, occupy or busy oneself, exercise; frolic; make sport of, mock; perform music," from West Germanic *plegan "occupy oneself about" (source also of Old Saxon plegan "vouch for, take charge of," Old Frisian plega "tend to," Middle Dutch pleyen "to rejoice, be glad," German pflegen "take care of, cultivate"), from PIE root *dlegh- "to engage oneself," forming words in Celtic, Germanic, Slavic, and possibly Latin.

Meaning "to take part in a game" is from c. 1200. Opposed to work (v.) since late 14c. Related: Played; playing. To play up "emphasize" is from 1909; to play down "minimize" is from 1930; to play along "cooperate" is from 1929. To play with oneself "masturbate" is from 1896; play for keeps is from 1861, originally of marbles or other children's games with tokens. To play second fiddle in the figurative sense is from 1809 ("Gil Blas"). To play into the hands (of someone) is from 1705. To play the _______ card is attested from 1886; to play fair is from mid-15c. To play (something) safe is from 1911; to play favorites is attested from 1902. For play the field see field (n.).
play (n.): Old English plega (West Saxon), plæga (Anglian) "quick motion; recreation, exercise, any brisk activity" (the latter sense preserved in swordplay, etc.), from or related to Old English plegan (see play (v.)). Meaning "dramatic performance" is attested by early 14c., perhaps late Old English. Meaning "free or unimpeded movement" of mechanisms, etc., is from c. 1200. By early Middle English it could mean variously, "a game, a martial sport, activity of children, joke or jesting, revelry, sexual indulgence." Sporting sense "the playing of a game" first attested mid-15c.; sense of "specific maneuver or attempt" is from 1868. To be in play (of a hit ball, etc.) is from 1788. Play-by-play is attested from 1927. Play on words is from 1798. Play-money is attested from 1705 as "money won in gambling," by 1920 as "pretend money."
play-bill (n.): also playbill, 1670s, from play (n.) + bill (n.1).
play-day (n.): c. 1600, from play + day.
play-dough (n.): 1959, from play + dough.
play-list (n.): also playlist, 1975 in the radio station sense, from play (v.) + list (n.1).
play-pen (n.): also playpen, 1931, from play + pen (n.2).
play-time (n.): also playtime, 1660s in the recreational sense, from play (n.) + time (n.).
playbook (n.): also play-book, 1530s, "book of stage plays," from play (n.) + book (n.). Meaning "Book of football plays" recorded from 1965.
playboy (n.): 1829, "wealthy bon vivant," from play (v.) + boy. Fem. equivalent playgirl first recorded 1934. As the name of a U.S. based magazine for men, from December 1953.
player (n.): Old English plegere, agent noun from play (v.). Stage sense is from mid-15c. As a pimp's word for himself (also playa), attested from 1974. Player-piano attested from 1901.
playful (adj.): mid-13c., from play (v.) + -ful. Related: Playfully; playfulness.
playground (n.): 1780, from play (v.) + ground (n.). Old English had plegstow "village sports ground," literally "place for play."
playhouse (n.): late Old English pleghus; see play (n.) + house (n.).
playmate (n.): 1640s, "companion, playfellow," from play (v.) + mate (n.). The sexual sense is from 1954 and the launch of "Playboy" magazine.
plaything (n.): 1670s, from play (v.) + thing.
playwright (n.): 1680s (Ben Jonson used it 1610s as a mock-name), from play (n.) + wright (n.).
plaza (n.): 1830, from Spanish plaza "square, place," from Vulgar Latin *plattia, from Latin platea "courtyard, broad street" (see place (n.)).
plea (n.): early 13c., "lawsuit," from Anglo-French plai (late 12c.), Old French plait "lawsuit, decision, decree" (9c.), from Medieval Latin placitum "lawsuit," in classical Latin, "opinion, decree," literally "that which pleases, thing which is agreed upon," properly neuter past participle of placere (see please). Sense development seems to be from "something pleasant," to "something that pleases both sides," to "something that has been decided." Meaning "a pleading, an agreement in a suit" is attested from late 14c. Plea-bargaining is first attested 1963. Common pleas (early 13c.) originally were legal proceedings over which the Crown did not claim exclusive jurisdiction (as distinct from pleas of the Crown); later "actions brought by one subject against another."
plead (v.): mid-13c., "make a plea in court," from Anglo-French pleder, Old French plaidier, "plead at court" (11c.), from Medieval Latin placitare, from Late Latin placitum (see plea). Sense of "request, beg" first recorded late 14c. Related: Pleaded; pleading; pleadingly.
pleading (n.): late 13c., "the carrying on of a suit at court," verbal noun from plead (v.). Meaning "supplication, intercession" is from early 15c.
pleasance (n.): late 14c., from Old French plaisance "pleasure, delight, enjoyment," from plaisant (see pleasant).
pleasant (adj.): late 14c. (early 14c. as a surname), from Old French plaisant "pleasant, pleasing, agreeable" (12c.), present participle of plaisir "to please" (see please). Pleasantry has the word's modern French sense of "funny, jocular." Related: Pleasantly.
pleasantry (n.): "sprightly humor in conversation," 1650s, from French plaisanterie "joke, jest; joking, jesting," from plaisant (see pleasant). Related: Pleasantries.