Online Etymology Dictionary

please (v.): early 14c., "to be agreeable," from Old French plaisir "to please, give pleasure to, satisfy" (11c., Modern French plaire, the form of which is perhaps due to analogy of faire), from Latin placere "to be acceptable, be liked, be approved," related to placare "to soothe, quiet" (source of Spanish placer, Italian piacere), possibly from PIE *plak-e- "to be calm," via notion of still water, etc., from root *plak- (1) "to be flat" (see placenta).

Meaning "to delight" in English is from late 14c. Inverted use for "to be pleased" is from c. 1500, first in Scottish, and paralleling the evolution of synonymous like (v.). Intransitive sense (do as you please) first recorded c. 1500; imperative use (please do this), first recorded 1620s, was probably a shortening of if it please (you) (late 14c.). Related: Pleased; pleasing; pleasingly.

Verbs for "please" supply the stereotype polite word ("Please come in," short for may it please you to ...) in many languages (French, Italian), "But more widespread is the use of the first singular of a verb for 'ask, request' " [Buck, who cites German bitte, Polish proszę, etc.]. Spanish favor is short for hace el favor "do the favor." Danish has in this sense vær saa god, literally "be so good."
pleased (adj.): "satisfied, contented," late 14c., past participle adjective from please (v.).
pleaser (n.): 1520s, agent noun from please.
pleasurable (adj.): 1570s, from pleasure (n.) + -able. Related: Pleasurability; pleasurably. For sense, compare comfortable.
pleasure (n.): late 14c., "condition of enjoyment," from Old French plesir, also plaisir "enjoyment, delight, desire, will" (12c.), from noun use of infinitive plaisir (v.) "to please," from Latin placere "to please, give pleasure, be approved" (see please (v.)). Ending altered in English 14c. by influence of words in -ure (measure, etc.). Meaning "sensual enjoyment as the chief object of life" is attested from 1520s.
pleasure (v.): 1530s, "to take pleasure in;" 1550s as "give pleasure to," from pleasure (n.). Sexual sense by 1610s. Related: Pleasured; pleasuring.
pleasure-seeker (n.): from pleasure (n.) + agent noun from seek.
pleat (v.): 1560s, used as the verb version of plait (n.) and probably representing an alternative pronunciation. Related: Pleated; pleating.
pleat (n.): "a fold," 1580s, variant of plait (n.). With a gap in the printed record 17c.-18c., but probably it was in continuous oral use.
pleather (n.): by 1991, from plastic + leather.
pleb (n.): 1856 as a colloquial shortening of plebeian in the ancient Roman sense. West Point sense attested by 1851 (see plebe).
plebe (n.): also pleb, "member of the lowest class at a U.S. military academy," 1833, probably a shortened form of plebeian "one of the lower class," which in Latin also had the short form plebs or plebes.
plebeian (adj.): "of or characteristic of the lower class," 1560s in a Roman historical sense, from Latin plebeius "belonging to the plebs," earlier plebes, "the populace, the common people" (as opposed to patricians, etc.), also "commonality; the mass, the multitude; the lower class," from PIE *ple- (see pleio-). In general (non-historical) use from 1580s.
plebeian (n.): "member of the lowest class," 1530s, from Latin plebius "person not of noble rank," from adjective meaning "of the common people" (see plebeian (adj.)).
plebiscite (n.): "direct vote of the people," 1860 (originally in reference to Italian unification), from French plébiscite (1776 in modern sense, originally with reference to Switzerland), from Latin plebiscitum "a decree or resolution of the people," from plebs (genitive plebis) "the common people" (see plebeian (adj.)) + scitum "decree," noun use of neuter past participle of sciscere "to assent, vote for, approve," inchoative of scire "to know" (see science). Used earlier (1530s) in a purely Roman historical context. Related: Plebiscitary.
plectrum (n.): something used to pluck the strings of a musical instrument, 1620s, from Latin plectrum, from Greek plektron "thing to strike with" (pick for a lyre, cock's spur, spear point, etc.), from plek-, root of plessein "to strike" (see plague (n.)).
pled: past tense and past participle of plead (v.).
pledge (n.): mid-14c., "surety, bail," from Old French plege (Modern French pleige) "hostage, security, bail," probably from Frankish *plegan "to guarantee," from *pleg-, a West Germanic root meaning "have responsibility for" (source also of Old Saxon plegan "vouch for," Middle Dutch plien "to answer for, guarantee," Old High German pflegan "to care for, be accustomed to," Old English pleon "to risk the loss of, expose to danger;" see plight (v.)).

Meaning "allegiance vow attested by drinking with another" is from 1630s. Sense of "solemn promise" first recorded 1814, though this notion is from 16c. in the verb. Weekley notes the "curious contradiction" in pledge (v.) "to toast with a drink" (1540s) and pledge (n.) "the vow to abstain from drinking" (1833). Meaning "student who has agreed to join a fraternity or sorority" dates from 1901.
pledge (v.): c. 1400, "to promise" (something to someone), "to give over as security for repayment," also "promise faith to," from pledge (n.) and from Old French plegier, from plege (n.). From mid-15c. as "to stand surety for, be responsible for;" late 15c. as "to mortgage." Meaning "put (someone) under oath" is from 1570s; sense of "to solemnly promise or guarantee" is from 1590s, as is sense "to drink a toast." Related: Pledged; pledging.
Pleiades (n.): late 14c., the seven daughters of Atlas and Pleione, transformed by Zeus into seven stars, from Latin Pleiades, from Greek Pleiades, perhaps literally "constellation of the doves" from a shortened form of peleiades, plural of peleias "dove" (from PIE root *pel- "dark-colored, gray"). Or perhaps from plein "to sail," because the season of navigation begins with their heliacal rising.

Old English had the name from Latin as Pliade. Mentioned by Hesiod (pre-700 B.C.E.), only six now are visible to most people; on a clear night a good eye can see nine (in 1579, well before the invention of the telescope, the German astronomer Michael Moestlin (1550-1631) correctly drew 11 Pleiades stars); telescopes reveal at least 500. Hence French pleiade, used for a meeting or grouping of seven persons.
plein-air (adj.): 1894, from French phrase en plein air, literally "in the open air." The style developed among French impressionists c. 1870.
pleio-: also pleo-, word-forming element meaning "more," from comb. form of Greek pleion "larger, greater in quantity, the more part, very many" (comp. of polys "much"), from PIE *ple- (source also of Latin plere "to fill," plebes, "the populace, the common people;" Greek plethein "be full," pleres "full"), possibly a variant of *pele- (1) "to fill" (see poly-).
pleiotropy (n.): 1921, from German pleiotrop (1910), from Greek pleion "greater in quantity, the more part, very many," (see pleio-) + trope "turn, turning" (see trope). Related: Pleiotropic; pleiotropism.
Pleistocene (adj.): "pertaining to the glacial period," 1839, coined by Lyell from Greek pleistos "most" (superlative of polys "much;" see poly-) + -cene.
plenary (adj.): 1510s, earlier plenar (mid-13c.), from Old French plenier, from Medieval Latin plenarius "entire, complete," from Latin plenus "full, filled, greatly crowded; stout, pregnant; abundant, abounding; complete," from PIE *pele- (1) "to fill" (see poly-). Related: Plenarily.
plenipotentiary (adj.): 1640s, from French plénipotentiaire and directly from Medieval Latin plenipotentiarius "having full power," from Late Latin plenipotens, from Latin plenus "full" (see plenary) + potentem "powerful" (see potent). As a noun from 1650s.
plenitude (n.): early 15c., from Old French plenitude and directly from Latin plenitudinem (nominative plenitudo) "abundance, completeness, fullness," from plenus "complete, full" (see plenary).
plenteous (adj.): c. 1300, plentivous, from Old French plentiveus "fertile, rich" (early 13c.), from plentif "abundant," from plentee "abundance" (see plenty). Related: Plentifully; plentifulness.
plentiful (adj.): late 15c., from plenty + -ful. Related: Plentifully.
plentitude (n.): 1610s, erroneous form of plenitude.
plenty (n.): mid-13c., "as much as one could desire," from Old French plentee, earlier plentet "abundance, profusion" (12c., Modern French dialectal plenté), from Latin plenitatem (nominative plenitas) "fullness," from plenus "complete, full" (see plenary). Meaning "condition of general abundance" is from late 14c. The colloquial adverb meaning "very much" is first attested 1842. Middle English had parallel formation plenteth, from the older Old French form of the word.
plenum (n.): 1670s, "filled space" (opposite of vacuum), from Latin plenum (spatium) "full (space)," neuter of adjective plenus "complete, full" (see plenary). The meaning "of a full assembly of legislators" is first recorded 1772.
pleo-: see pleio-.
pleomorphic (adj.): "having more than one form," 1886, from pleo- + -morphe "form" (see Morpheus). Related: Pleomorphism.
pleonasm (n.): "redundancy in words," 1580s, from Late Latin pleonasmus, from Greek pleonasmos, from pleonazein "to be more than enough, to be superfluous," in grammatical use, "to add superfluously," from comb. form of pleon "more" (see pleio-).
pleonastic (adj.): 1778, with -ic + Greek pleonastos "abundant," from pleonazein (see pleonasm). Related: Pleonastical (1650s).
plesiosaurus (n.): 1825, from Modern Latin Pleisiosaurus (1821), coined by English paleontologist William Daniel Conybeare (1787-1857) from Greek plesios "near," related to pelas, + -saurus.
plethora (n.): 1540s, a medical word for "excess of body fluid," from Late Latin plethora, from Greek plethore "fullness," from plethein "be full" (see pleio-). Figurative meaning "too-muchness, overfullness in any respect" is first recorded 1700. Related: Plethoric.
pleura (n.): early 15c., medical Latin, from Greek pleura "side of the body, rib," also "flank of an army, page of a book," of unknown origin.
pleural (adj.): 1835, from pleura + -al (1). Alternative pleuric is attested from 1825.
pleurisy (n.): late 14c., from Old French pleurisie (13c., Modern French pleurésie) and directly from Late Latin pleurisis "pleurisy," alteration of Latin pleuritis "pain in the side," from Greek pleuritis, from pleura "side of the body, rib," of unknown origin. Spelling altered in Late Latin on model of Latin stem plur- "more" (as in Medieval Latin pluritas "multitude"), as if in reference to "excess of humors."
pleuro-: before vowels pleur-, word-forming element meaning "pertaining to the side; pertaining to the pleura," from comb. form of Greek pleura (see pleura).
plex: in various usages, from Latin plex-, past participle stem of plectere "to plait" (see complex (adj.)).
Plexiglas (n.): 1935, proprietary name (Röhm & Haas) for a substance also sold as Perspex and Lucite. Often written incorrectly as plexiglass.
plexus (n.): 1680s, Modern Latin, literally "braid, network," noun use of past participle of Latin plectere "to twine, braid, fold" (see complex (adj.)); used of a network, such as solar plexus "network of nerves in the abdomen" (see solar). Related: Plexal.
pliable (adj.): late 14c., from Old French ploiable "flexible, bendable," from plier "to bend" (see ply (n.)). Related: Pliably, pliability.
pliant (adj.): late 14c., from Old French ploiant "bending, supple; compliant, fickle," as a noun, "turncoat" (13c.), present participle of ploier "to bend" (see ply (n.)). Figurative sense of "easily influenced" is from c. 1400. Related: Pliancy.
plie (n.): in ballet, 1892, from French plié, from plier literally "to bend," from Old French ploier (see ply (n.)).
plier (n.): "one who folds," 1670s, agent noun from ply (v.).
pliers (n.): "pincers," 1560s, plural agent noun from ply (n.). French cognate plieur meant "folder."