Online Etymology Dictionary

equilateral (adj.): "having all sides equal," 1560s, from Late Latin aequilateralis, from aequi- (see equal (adj.)) + lateralis (see lateral). Related: Equilaterally.
equilibrium (n.): c. 1600, "state of mental balance," from Latin aequilibrium "an even balance; a horizontal position," from aequilibris "equal, level, horizontal, evenly balanced," from aequus "equal" (see equal (adj.)) + libra "a balance, pair of scales, plummet" (see Libra). Related: Equilibrious.
equine (adj.): 1765, from Latin equinus "of a horse, of horses; of horsehair," from equus "horse," from PIE root *ekwo- "horse" (source also of Greek hippos, Old Irish ech, Old English eoh, Gothic aihwa-, Sanskrit açva-, Avestan aspa-, Old Church Slavonic ehu-, all meaning "horse").
equinox (n.): c. 1400, "point at which the sun crosses the earth's equator, making day and night of equal length everywhere," from Old French equinoce (12c.) or directly from Medieval Latin equinoxium "equality of night (and day)," from Latin aequinoctium, usually in plural, dies aequinoctii "the equinoxes," from aequus "equal" (see equal (adj.)) + nox (genitive noctis) "night" (see night). The Old English translation was efnniht. Related: Equinoctial.
equip (v.): 1520s, from Middle French équiper "to fit out," from Old French esquiper "fit out a ship, load on board" (12c.), probably from Old Norse skipa "arrange, place in order," usually "fit out a ship," but also of warriors manning a hall and trees laden with ripe fruit, from skip "ship" (see ship (n.)). Related: Equipped; equipping. Similar words in Spanish and Portuguese ultimately are from Germanic.
equipage (n.): 1570s, from French équipage (15c.), from équiper "to fit out" (see equip). Now largely replaced by equipment. In 18c. often especially tweezers, a toothpick, earpick, nail-cleaner, etc., carried on the person in a small case.
equiparation (n.): mid-15c., "impartial treatment;" 1610s, "equal ranking;" from Latin aequiparationem (nominative aequiparatio) "an equalizing, comparison," from past participle stem of aequiparare "put on equality, compare," from aequipar "equal, alike," from aequus "equal, even" (see equal (adj.)) + par (see par (n.)). Related: Equiparate.
equipment (n.): 1717, "things equipped;" 1748, "action of equipping;" from equip + -ment, or from French équipement. Superseding earlier equipage.
equipoise (n.): "an equal distribution of weight," 1650s, a contraction of the phrase equal poise (1550s); see equal (adj.) + poise (n.).
equitable (adj.): 1640s, from French équitable (16c.), from équité (see equity). Related: Equitably.
equity (n.): early 14c., "quality of being equal or fair, impartiality in dealing with others," from Old French equite (13c.), from Latin aequitatem (nominative aequitas) "equality, uniformity, conformity, symmetry; fairness, equal rights; kindness, moderation," from aequus "even, just, equal" (see equal (adj.)). As the name of a system of law, 1590s, from Roman naturalis aequitas, the general principles of justice which corrected or supplemented the legal codes.
equivalence (n.): 1540s, from French équivalence, from Medieval Latin aequivalentia, from Late Latin aequivalentem "equivalent" (see equivalent). Related: Equivalency (1530s).
equivalent (adj.): early 15c., from Middle French equivalent and directly from Late Latin aequivalentem (nominative aequivalens) "equivalent," present participle of aequivalere "be equivalent," from Latin aequus "equal" (see equal (adj.)) + valere "be well, be worth" (see valiant). As a noun from c. 1500, "that which is equal or corresponds to." Related: Equivalently.
equivocal (adj.): c. 1600, from Late Latin aequivocus "of equal voice, of equal significance, ambiguous" (see equivocation) + -al (1). Earlier in same sense was equivoque (late 14c.). Related: Equivocally (1570s).
equivocate (v.): early 15c., equivocaten, from Medieval Latin equivocatus, past participle of equivocare "to call by the same name, be called by the same name, have the same sound," from Late Latin aequivocus "of identical sound" (see equivocation). Related: Equivocated; equivocating.
equivocation (n.): late 14c., "fallacy of using a word in different senses at different stages of the reasoning" (a loan-translation of Greek homonymia, literally "having the same name"), from Old French equivocation, from Late Latin aequivocationem (nominative aequivocatio), noun of action from aequivocus "of identical sound, of equal voice, of equal significance, ambiguous, of like sound," past participle of aequivocare, from aequus "equal" (see equal (adj.)) + vocare "to call" (see voice (n.)).
equivocator (n.): 1590s, from Late Latin aequivocator, agent noun from aequivocare (see equivocation).
equus (n.): "a horse," Latin, see equine.
er: as a sound of hesitation or uncertainty, attested from mid-19c.
ER: abbreviation of emergency room, by 1965.
era (n.): 1716, earlier aera (1610s), from Late Latin aera, era "an era or epoch from which time is reckoned" (7c.), probably identical with Latin aera "counters used for calculation," plural of aes (genitive aeris) "brass, copper, money" (see ore, also compare copper). The Latin word's use in chronology said to have begun in 5c. Spain (where the local era, aera Hispanica, began 38 B.C.E.; some say because of a tax levied that year). Other ancient eras included the Chaldean (autumn of 311 B.C.E.), the Era of Actium (31 B.C.E.), of Antioch (49 B.C.E.), of Tyre (126 B.C.E.), the Olympiadic (July 1, 776 B.C.E.) and the Seleucidan (autumn 312 B.C.E.). In English it originally meant "the starting point of an age" (compare epoch); meaning "system of chronological notation" is from 1640s; that of "historical period" is from 1741, as in the U.S. Era of Good Feeling (1817) was anything but.
eradicate (v.): early 15c., "destroy utterly," literally "pull up by the roots," from Latin eradicatus, past participle of eradicare "to root out, annihilate" (see eradication). Related: Eradicated; eradicating; eradicable.
eradication (n.): early 15c., from Latin eradicationem (nominative eradicatio), noun of action from past participle stem of eradicare "root out, extirpate, annihilate," from assimilated form of ex- "out" (see ex-) + radix (genitive radicis) "root" (see radish).
erasable (adj.): 1829, from erase + -able.
erase (v.): c. 1600, from Latin erasus, past participle of eradere "scrape out, scrape off, shave; abolish, remove," from assimilated form of ex- "out" (see ex-) + radere "to scrape" (see raze). Of magnetic tape, from 1945. Related: Erased; erasing.
eraser (n.): "thing that erases writing," 1790, American English, agent noun from erase. Originally a knife for scraping off the ink. As a rubber product for removing pencil marks, from 1858.
Erasmus: masc. proper name, Latin, literally "beloved;" related to Greek erasmios "lovely, pleasant," from eran "to love" (see Eros). Related: Erasmian.
Erastus: masc. proper name, Latin, literally "beloved," from Greek erastos, verbal adjective of eran "to love" (see Eros).
erasure (n.): "an erasing, an obliterating," 1734, from erase + -ure.
Erato: muse who presided over lyric poetry, literally "the Lovely," from Greek erastos "loved, beloved; lovely, charming," verbal adjective of eran "to love, to be in love with" (see Eros).
erbium (n.): 1843, coined in Modern Latin with metallic element name -ium + erbia, name given by Swedish chemist Carl Gustaf Mosander (1797-1858), who discovered it, from second element in Ytterby, name of a town in Sweden where mineral containing it was found (see Ytterbium).
ere (prep.): c. 1200, from Old English ær (adv., conj., & prep.) "soon, before (in time)," from Proto-Germanic *airiz, comparative of *air "early" (source also of Old Saxon, Old Frisian, Old High German er, Dutch eer; German eher "earlier;" Old Norse ar "early;" Gothic air "early," airis "earlier"), from PIE *ayer- "day, morning" (source also of Avestan ayar "day;" Greek eerios "at daybreak," ariston "breakfast"). The adverb erstwhile retains the Old English superlative ærest "earliest."
Erebus: in Homer, etc., the place of darkness between Earth and Hades, from Latin Erebus, from Greek Erebos, which is of unknown origin, perhaps from Semitic (compare Hebrew erebh "sunset, evening"), or from PIE *regw-es- "darkness" (source also of Sanskrit rajas "the atmosphere, thick air, mist, darkness;" Gothic rikwis "darkness"). Used figuratively of darkness from 1590s.
Erechtheus: legendary first king and founder of Athens, from Latin Erechtheus, from Greek Erekhtheos, literally "render, shaker" (of the earth), from erekhthein "to rend, break, shatter, shake." Hence Erechtheum, the name of a temple on the Athenian acropolis.
erect (adj.): late 14c., "upright, not bending," from Latin erectus "upright, elevated, lofty; eager, alert, aroused; resolute; arrogant," past participle of erigere "raise or set up," from e- "up, out of" + regere "to direct, keep straight, guide" (see regal).
erect (v.): c. 1400, a back-formation from erect (adj.) or else from Latin erectus. Related: Erected; erecting.
erectile (adj.): 1822, "pertaining to muscular erection," from French érectile, from Latin erect-, past participle stem of erigere "to raise or set up" (see erect (adj.)).
erection (n.): mid-15c., "establishment; advancement," from Late Latin erectionem (nominative erectio), noun of action from past participle stem of erigere "to set up, erect" (see erect (adj.)). Meanings "the putting up" (of a building, etc.), "stiffening of the penis" (also sometimes of the turgidity and rigidity of the clitoris) are both from 1590s.
erector (n.): 1530s, "one who builds," agent noun in Latin form from erect (v.). In reference to muscles from 1831. The children's buildig kit Erector (commonly known as an Erector set) was sold from 1913.
eremite (n.): c. 1200, learned form of hermit (q.v.) based on Church Latin eremita. Since mid-17c. in poetic or rhetorical use only, except in reference to specific persons in early Church history. Related: Eremitic; eremitical.
Erewhon (n.): "utopia," from title of a book published 1872 by British author Samuel Butler (1835-1902), a partial reversal of nowhere.
erg (n.1): unit of energy in the C.G.S. system, coined 1873 by the British Association for the Advancement of Science, from Greek ergon "work" (see organ).
erg (n.2): "region of drifting sand dunes," 1875, from French erg (1854), from North African Arabic 'irj, from a Berber word.
ergative (adj.): 1943, in reference to grammatical case used for the subjects of transitive verbs (in Eskimo, Basque, Caucasian languages), from Greek ergatos "workman," from comb. form of ergos "work" (see organ) + -ive.
ergo (conj.): c. 1400, from Latin ergo "therefore, in consequence of," possibly from *ex rogo "from the direction," from ex "out of" (see ex-) + root of regere "to direct, to guide" (see regal). Used in logic to introduce the conclusion of a complete and necessary syllogism.
ergonomics (n.): "scientific study of the efficiency of people in the workplace," coined 1950 from Greek ergon "work" (see organ) + second element of economics.
ergophobia (n.): "fear of work," 1905, coined by British medical man Dr. William Dunnett Spanton, from comb. form of Greek ergos "work" (see organ) + -phobia "fear."
Mr. W.D. Spanton (Leeds) considered that the most prominent causes of physical degeneration were--efforts to rear premature and diseased infants, absurd educational high pressure, cigarette smoking in the younger generation, and late hours at night; in fact, the love of pleasure and ergophobia in all classes of society. He considered that there was too much cheap philanthropy, that life was made too easy for the young poor, and that by modern educational methods proper parental discipline was rendered almost impossible. [report on the 73rd annual meeting of the British Medical Association, "Nature," Aug. 3, 1905]
ergot (n.): fungal disease of rye and other grasses, 1680s, from French ergot "ergot," also "a spur, the extremity of a dead branch," from Old French argot "cock's spur" (12c.), which is of unknown origin. The blight so called from the shape the fungus forms on the diseased grain. Related: Ergotic. An alkaloid from the fungus, ergotamine (1921) is used to treat migraines.
ergotism (n.): "disease caused by eating ergot-infected breadstuffs," 1816; see ergot + -ism.
Eric: masc. proper name, from Old Norse Eirikr, literally "honored ruler," from Proto-Germanic *aiza- "honor" + *rik- "ruler" (see regal). The German form is Erich.