Online Etymology Dictionary

pylorus (n.): 1610s, from Late Latin pylorus "the lower orifice of the stomach," from Greek pyloros, literally "gatekeeper, porter," from pyle "gate" (see pylon) + ouros "watcher, guardian" (see warrant (n.)). Related: Pyloric.
pyo-: word-forming element meaning "pus," from comb. form of Greek puon "pus" (see pus).
pyogenic (adj.): 1840s, from pyogenesis, medical Latin, from pyo- "pus" + -genic "producing."
Pyongyang: North Korean capital, from Korean p'yong "flat" + yang "land."
pyracanth (n.): thorny shrub of the apple family, 1660s, from Modern Latin genus name Pyracantha, from Greek pyrakantha (Dioscorides), a plant named but not described, from pyr "fire," from PIE root *paəwr- "fire" (see fire (n.)) + akantha "thorn, thorny plant," from PIE root *ak- "sharp, pointed" (see acrid).
pyramid (n.): 1550s (earlier in Latin form piramis, late 14c.), from French pyramide (Old French piramide "obelisk, stela," 12c.), from Latin pyramides, plural of pyramis "one of the pyramids of Egypt," from Greek pyramis (plural pyramides) "a pyramid," apparently an alteration of Egyptian pimar "pyramid." Financial sense is from 1911. Related: Pyramidal.
pyre (n.): 1650s, from Latin pyra and directly from Greek pyra "funeral pyre; altar for sacrifice; any place where fire is kindled," from pyr "fire," cognate with Old English fyr (see fire (n.)).
Pyrenees: 1550s, from French Pyrénées, from Latin Pyrenæi montes, from Greek Pyrene, name of a daughter of Bebryx/Bebrycius who was beloved of Herakles; she is said to be buried in these mountains (or that the mountains are the tomb Herakles reared over her corpse). The name is said to mean literally "fruit-stone," but Room says it might be Greek pyr "fire" + eneos "dumb, speechless," which perhaps translates or folk-etymologizes a Celtic goddess name. "In medieval times there was no overall name for the range and local people would have known only the names of individual mountains and valleys" [Room, Adrian, Place Names of the World, 2nd ed., McFarland & Co., 2006]. Related: Pyrenean.
pyretic (adj.): 1809, from French pyrétique or directly from Modern Latin pyreticus, from Greek pyretos "fever, burning heat," related to pyr "fire," from PIE root *paəwr- "fire" (see fire (n.)). As a noun, "a pyretic agent," from 1728.
Pyrex (n.): 1915, proprietary name (Corning Glass Works, Corning, N.Y.), arbitrary coinage, in which eager etymologists see implications of Greek pyr "fire" and perhaps Latin rex "king;" but the prosaic inventors say it was based on pie (n.1), because pie dishes were among the first products made from it. The -r- is purely euphonious.
pyrexia (n.): "fever," 1769, medical Latin, from Greek pyrexis "feverishness," from pyressein "to be feverish, to be ill of fever," from pyretos "fever, burning heat," related to pyr "fire," from PIE root *paəwr- "fire" (see fire (n.)) + abstract noun ending -ia.
pyrgologist (n.): "one versed in the structure and history of towers," 1877, from Greek pyrgos "a tower; highest point of a building" + -ologist. It seems to have been used once, in the "Athenaeum" of Aug. 18, and then forgotten except in the dictionary.
pyrite (n.): "metallic iron disulfide, fool's gold," 1550s, from Old French pyrite (12c.), from Latin pyrites, from Greek pyrites lithos "stone of fire, flint" (so called because it glitters), from pyrites "of or in fire," from pyr (genitive pyros) "fire," from PIE root *paəwr- "fire" (see fire (n.)). Related: Pyritic.
pyro-: before vowels pyr-, word-forming element form meaning "fire," from Greek pyro-, combining form of pyr (genitive pyros) "fire, funeral fire," also symbolic of terrible things, rages, "rarely as an image of warmth and comfort" [Liddell & Scott], from PIE root *paəwr- "fire" (see fire (n.)). Pyriphlegethon, literally "fire-blazing," was one of the rivers of Hell.
pyroclastic (adj.): 1887, from pyro- + clastic.
pyrogen (n.): 1858, as a proposed word for "electricity," from pyro- + -gen. Meaning "fever-producer" is from 1896. Related: Pyrogenic; pyrogenetic. Greek pyrogenes meant "born in fire, wrought by fire."
pyrolatry (n.): "fire-worship," 1660s, from pyro- + -latry. Related: Pyrolater.
pyrolusite (n.): "manganese dioxide," 1828, the name given in Roman times, when it was used, in a heated state, to de-colorize glass, from Greek elements pyro- "by heat, by fire" (see pyro-) + lysis "a loosening" (see lose).
pyrolysis (n.): 1879, from pyro- + -lysis. Related: Pyrolytic.
pyromania (n.): 1840, from pyro- "fire" + mania "madness, frenzy." Used in German in the 1830s.
The propensity which leads an insane person to accomplish his purpose by burning, has been considered to merit particular notice, and to constitute a variety of monomania. Dr. Marc, of France, has published a memoir on the subject; he gives the name of pyromania to it, and considers that, like other insane propensities, it may be the result of instinct, or it may be the result of delusion--reasoning upon erroneous principles. [Alexander Morrison, M.D., "The Physiognomy of Mental Diseases," London, 1840]
An older word for it was incendiarism.
pyromaniac (adj.): 1855, from pyromania. As a noun from 1861.
pyrophobia (n.): "morbid fear or fire," 1871, from pyro- + -phobia.
pyrophoric (adj.): 1779, from Modern Latin pyrophorus, literally "fire-bearing," from Greek pyrophoros, from pyro- (see pyro-) + phoros "bearer," from pherein "to carry" (see infer). Related: Pyrophorous; pyrophorus.
pyrotechnic (adj.): 1704, "of or pertaining to fire;" 1825, "of or pertaining to fireworks," from pyro- + Greek tekhnikos "made by art," from tekhne "art" (see techno-). Figurative use attested from 1847. Related: Pyrotechnical (1610s, from pyrotechny "use of gunpowder," 1570s).
pyrotechnician (n.): 1729, from pyrotechnic + -an.
pyrotechnics (n.): 1729, from pyrotechnic (also see -ics). Figurative sense is from 1901.
Pyrrhic (adj.): 1885 (usually in phrase Pyrrhic victory), from Pyrrhus, king of Epirus, who defeated Roman armies at Asculum, 280 B.C.E., but at such cost to his own troops that he was unable to follow up and attack Rome itself, and is said to have remarked, "one more such victory and we are lost."
pyrrhic (n.): "dance in armor" (1590s), also a type of metrical foot (1620s), from Latin pyrrhicha, from Greek pyrrikhe orkhesis, the war-dance of ancient Greece, traditionally named for its inventor, Pyrrikhos. The name means "reddish," from pyrros "flame-colored," from pyr "fire," from PIE root *paəwr- "fire" (see fire (n.)). As an adjective from 1749.
Pyrrhonic (adj.): 1590s, "pertaining to Pyrrho" (c. 360-c. 275 B.C.E.), skeptic philosopher of Elis, who held the impossibility of attaining certainty of knowledge. Related: Pyrrhonism; Pyrrhonist.
Pythagorean (adj.): 1540s, from Latin Pythagoreus "of or pertaining to Pythagoras," Greek philosopher of Samos (6c. B.C.E.), whose teachings included transmigration of the soul and vegetarianism (these are some of the commonest early allusions in English). The Pythagorean theorem is the 47th of the first book of Euclid.
Pythia (n.): "priestess of Apollo at Delphi," 1842, from Greek pythia (hiereia) "(Priestess) of Pythian Apollo, from a variant form of Pythios, an epithet of Apollo, from Pytho, older name of the region of Delphi (see python).
Pythian (adj.): c. 1600, "pertaining to Delphi or Delphic Apollo," from Pythia + -an. As a noun from 1590s.
python (n.): 1580s, fabled serpent, slain by Apollo near Delphi, from Latin Python, from Greek Python "serpent slain by Apollo," probably related to Pytho, the old name of Delphi, perhaps itself related to pythein "to rot," or from PIE *dhubh-(o)n-, from *dheub- "hollow, deep, bottom, depths," and used in reference to the monsters who inhabit them. Zoological application to large non-venomous snakes of the tropics is from 1836, originally in French.
Pythonesque (adj.): 1975, in reference to the style of humor popularized by British TV series "Monty Python's Flying Circus."
pythoness (n.): late 14c., "woman with the power of soothsaying," from Old French phitonise (13c.), from Late Latin pythonissa, used in Vulgate of the Witch of Endor (I Sam. xxviii:7), and often treated as her proper name, literally fem. of pytho "familiar spirit;" which ultimately is connected with the title of the prophetess of the Delphic Oracle, Greek pythia hiereia, from Pythios, an epithet of Apollo, from Pytho, older name of the region of Delphi (see python).
pyuria (n.): 1811, from pyo- + -uria (see urine).
pyx (n.): c. 1400, "a box," especially the vessel in which the host or consecrated bread is preserved, from Latin pyxis, from Greek pyxis "box-wood; a box," from pyxos "box-wood; box-tree," of uncertain origin.