araneology (n.)
"study of spiders," 1798, from araneae, zoological name of the order of spiders, from Latin aranea "spider" (source French (araignée, Spanish (araña, Italian aragna; see arachnid) + -ology. Related: Araneous (ad.), 1650s; araneidan (n.), 1835; araneologist.
arational (adj.)
also a-rational, "not purporting to be governed by laws of reason," 1935; see a- (2) + rational.
Araucanian (n.)
language spoken by the Araucanian people of central Chile, 1809, also Araucano, Mapudungu.
Arawakan (n.)
language group formerly widespread in the West Indies and South America, 1910, from the self-designation of the Arawak people on continental South America. They were identical with, or closely related to the natives whom Columbus encountered on the islands, who were historically called Taino.
arbalest (n.)
also arbalist, type of crossbow, c. 1300, from Old French arbaleste "large crossbow with a crank" (12c., Modern French arbalète), from Vulgar Latin arbalista, from Late Latin arcuballista "catapult," from Latin arcus "bow" (see arc (n.)) + ballista "machine for throwing projectiles" (from PIE root *gwele- "to throw, reach"). German Armbrust is from the same French word but mangled by folk etymology. Related: Arbalester.
The missile of the arbalist was discharged with such force as to penetrate ordinary armor, and the weapon was considered so deadly as to be prohibited by a council of the church except in warfare against infidels.
arbiter (n.)
late 14c., "person who has power of judging absolutely according to his own pleasure in a dispute or issue," from Old French arbitre "arbiter, judge" (13c.) and directly from Latin arbiter "one who goes somewhere (as witness or judge)," in classical Latin used of spectators and eye-witnesses; specifically in law, "he who hears and decides a case, a judge, umpire, mediator;" from ad "to" (see ad-) + baetere "to come, go."

The specific sense of "one chosen by two disputing parties to decide the matter" is from 1540s. Compare Arbitrator. The earliest form of the word attested in English is the fem. noun arbitress (mid-14c.) "a woman who settles disputes." Gaius Petronius Arbiter (circa 27-66 C.E.) was a friend of Nero, noted voluptuary, reputed author of the "Satyricon," and an authority on matters of taste and style (elegantiae arbiter, punning on the name).
arbitrage (n.)
"arbitration, exercise of the function of an arbitrator," late 15c., from Old French arbitrage "arbitration, judgment," from arbitrer "to arbitrate, judge," from Late Latin arbitrari, from Latin arbiter "judge, umpire, mediator" (see arbiter). In finance, "the business founded on a calculation of the temporary differences in the price of securities in different markets" (1875).
arbitral (adj.)
"pertaining to arbitration" (without the negative overtones of arbitrary), c. 1600; see arbiter + -al (1).
arbitrary (adj.)
c. 1400, "deciding by one's own discretion, depending on one's judgment," from Latin arbitrarius "of arbitration," hence "depending on the will, uncertain," from arbiter (see arbiter). The meaning in English gradually descended to "capricious, ungoverned by reason or rule, despotic" (1640s). Related: Arbitrarily; arbitrariness.
arbitrate (v.)
1580s, "act as an umpire, mediate, decide, determine, give an authoritative decision," from Latin arbitratus, past participle of arbitrari "be of an opinion, give a decision," from arbiter "a judge, umpire, mediator" (see arbiter). Meaning "act as an arbitrator" is from 1610s. Related: Arbitrated; arbitrating; arbitrable (1530s). The earlier verb form was arbitren "decide a dispute by arbitration" (early 15c.).
arbitration (n.)
late 14c., "faculty of making a choice or decision, judgment, discretion;" early 15c., "authority or responsibility for deciding a dispute," from Old French arbitracion and directly from Latin arbitrationem (nominative arbitratio) "judgment, will," noun of action from past participle stem of arbitrari "to be of an opinion, give a decision," from arbiter "a judge, umpire, mediator" (see arbiter). Meaning "settlement of a dispute by a third party" is from 1630s. Related: Arbitrative.
arbitrator (n.)
"person chosen by opposite parties to decide some point at issue between them," early 15c., from Late Latin arbitrator "a spectator, hearer, witness; a judge," agent noun from past participle stem of arbitrari "be of an opinion, give a decision," from arbiter "a judge, umpire, mediator" (see arbiter).

The legal form of popular arbiter. In modern usage, an arbiter makes decisions of his own accord and is accountable to no one but himself; an arbitrator decides issues referred to him by the parties. "It is often the practice to appoint two or more arbitrators, with an umpire, chosen usually by them, as final referee" [OED].
arbitrer (n.)
"arbitrator," late 14c., from Anglo-French arbitrour, Old French arbitreor "arbitrator, judge" (13c.), from Old French arbitrer "a judge, umpire, mediator" (see arbiter). Fem. form arbitress is from mid-14c.
arbor (n.1)
c. 1300, herber, "herb garden, pleasure garden," from Old French erbier "field, meadow; kitchen garden," from Latin herba "grass, herb" (see herb). Later "a grassy plot" (mid-14c., a sense also in Old French), "shaded nook, bower formed by intertwining of trees, shrubs, or vines" (mid-14c.).

Probably not from Latin arbor "tree" (see arbor (n.2)) though perhaps influenced by its spelling; the change from er- to ar- before consonants in Middle English also reflects a pronunciation shift: compare farm from ferme, harbor from Old English herebeorg.
arbor (n.2)
"main support or beam of a machine," 1650s, from Latin arbor, arboris "tree," from Proto-Italic *arthos, which de Vaan derives from PIE *herdhos "height, uprightness," from root *eredh- "to grow, high" (see ortho-).
Arbor Day
day set aside in U.S. "for planting forest trees to make lumber for the generations yet to come" ["Congressional Record," June 1892], first celebrated April 10, 1872, in Nebraska (a largely treeless state), the brainchild of U.S. agriculturalist and journalist J. Sterling Morton (1832-1902). From Latin arbor, arboris "tree" (see arbor (n.2)).
arbor vitae (n.)
also arbor-vitae, type of evergreen shrub, 1660s, name given by French physician and botanist Charles de Lécluse (1525-1609), Latin, literally "tree of life;" see arbor (n.2) + vital. Also used in late 18c. rogue's slang as a cant word for "penis."
arboreal (adj.)
1660s, "pertaining to trees," from Latin arboreus "pertaining to trees," from arbor, arboris "tree" (see arbor (n.2)) + -al (1). From 1834 as "living in or among trees."
arboretum (n.)
"tree-garden, place where trees or shrubs are cultivated," 1838, from Latin arboretum, literally "a place grown with trees," from arbor "tree" (see arbor (n.2)) + -etum, suffix used to form the names of gardens and woods.
arboricide (n.)
"wanton destruction of trees," 1853, from Latin arbor "tree" + ending from suicide, etc. Meaning "one who wantonly cuts down trees" is from 1873. Related: Arboricidal (1865).
Arboricide is a crime, as well as homicide. The name of Gastrell, who cut down Shakspeare's mulberry tree, is justly followed by the execrations of posterity, and hangs forever on a gibbet of reproach, vainly craving the boon of oblivion. ["New England Farmer," March 1853]
arboriculture (n.)
"the are of planting, training, and trimming trees and shrubs," 1822, from Latin arbor, arboris "tree" (see arbor (n.2)) + -culture, abstracted from agriculture. Perhaps modeled on French arboriculture (by 1808). Related: Arboricultural; arboriculturist (1825).
arborist (n.)
1570s, from Latin arbor "tree" (see arbor (n.2)) + -ist. In early use probably from French arboriste.
arbour (n.)
chiefly British English spelling of arbor (n.1); for spelling, see -or.
arc (v.)
1893, in the electrical sense, from arc (n.). Meaning "to move in an arc" attested by 1954. Related: Arced; arcing.
arc (n.)
late 14c., "part of a curved line," originally in reference to the sun's apparent motion across the sky, from Old French arc "bow, arch, vault" (12c.), from Latin arcus "a bow, arch," from Proto-Italic *arkwo- "bow."

This has Germanic cognates in Gothic arhvazna, Old English earh, Old Norse ör "arrow," from Proto-Germanic *arkw-o- "belonging to a bow." It also has cognates in Greek arkeuthos, Latvian ercis "juniper," Russian rakita, Czech rokyta, Serbo-Croatian rakita "brittle willow." De Vaan sees an Italo-Germanic word for "bow" which can be connected with Balto-Slavic and Greek words for "willow" and "juniper" "under the well-founded assumption that the flexible twigs of juniper or willow were used as bows." The Balto-Slavic and Greek forms point to *arku-; "as with many plant names, this is likely to be a non-IE loanword." Electrical sense is from 1821.
arc-light (n.)
"light produced by an electric arc," 1871, from arc (n.) + light (n.). Related: Arc-lamp.
arcade (n.)
1731, "vaulted space" (as arcado from 1640s), via French arcade, which probably is from Italian arcata "arch of a bridge," from arco "arc," from Latin arcus "a bow, arch" (see arc (n.)).

The English word was applied to passages formed by a succession of arches supported on piers or pillars, avenues of trees, and ultimately to any covered avenue (1731), especially one lined with shops (1829) or amusements; hence arcade game (1977).
Arcadia
mountainous district in central Peloponnesus, a Latinized form of Greek Arkadia, which is traditionally from Arkas (genitive Arkadas), son of Zeus, name of the founder and first ruler of Arcadia.

The idealized Arcadia of later pastoral romance, "the home of piping shepherds and coy shepherdesses, where rustic simplicity and plenty satisfied the ambition of untutored hearts, and where ambition and its crimes were unknown" [John Mahaffy, "History of Classical Greek Literature," 1880] seems to have been inspired by "Arcadia," a description of shepherd life in prose and verse by Italian Renaissance poet Iacopo Sannazaro, published in 1502, which went through 60 editions in the century. It is exemplified in English by Sir Philip Sidney's poem, published in 1590, and in Spanish by Lope de Vega's, printed in 1598. Classical Arcadia, Mahaffy writes:
was only famed for the marketable valour of its hardy mountaineers, of whom the Tegeans had held their own even against the power of Sparta, and obtained an honourable place in her army. It was also noted for rude and primitive cults, of which later men praised the simplicity and homely piety--at times also, the stern gloominess, which did not shrink from the offering of human blood. ["Rambles and Studies in Greece," 1887]
Poetic Arcady is from 1580s.
Arcadian (adj.)
"ideally rustic or rural;" as a noun, "an idealized rustic," 1580s, from Greek Arkadia, a mountainous district landlocked in the Peloponnesus, regarded by the ancient Greeks as rude, impoverished, and inhospitable, but taken by 16c. European poets as an ideal region of rural felicity. See Arcadia.
arcana (n.)
"hidden things, mysteries," 1590s, a direct adoption of the Latin plural of arcanum "a secret, a mystery," an important word in alchemy, from neuter of adjective arcanus "secret, hidden, private, concealed" (see arcane). Occasionally mistaken for a singular and pluralized as arcanas, because arcana is far more common than arcanum.
arcane (adj.)
1540s, from Latin arcanus "secret, hidden, private, concealed," from arcere "to close up, enclose, contain," from arca "chest, box, place for safe-keeping," from PIE root *ark- "to hold, contain, guard" (source also of Greek arkos "defense," arkein "to ward off;" Armenian argel "obstacle;" Lithuanian raktas "key," rakinti "to shut, lock").
arcanum (n.)
proper singular form of arcana (q.v.); in alchemy, a supposed great secret of nature.
arch (v.)
early 14c., "to form an arch" (implied in arched); c. 1400 in transitive sense "furnish with an arch," from arch (n.). Related: Arching.
arch (n.)
"structure (in a building, bridge, etc.) in the shape of a curve that stands when supported only a the extremities," c. 1300, from Old French arche "arch of a bridge, arcade" (12c.), from Latin arcus "a bow" (see arc (n.)). Replaced native bow (n.1).

Originally architectural in English; transferred by early 15c. to anything having a curved form (eyebrows, feet, etc.). The commemorative or monumental arch is attested in English from late 14c. Compare Middle English Seinte Marie Chirche of the Arches (c. 1300) in London, later known as St. Mary-le-Bow, site of an ecclesiastical court, so called for the arches that supported its steeple (the modern church is by Sir Christopher Wren, rebuilt after the Great Fire of 1666).
arch (adj.)
1540s, "chief, principal," from separate use of the prefix arch-, which is attested from late Old English (in archangel, archbishop, etc.). The prefix figured in so many derogatory uses (arch-rogue, arch-knave, etc.) that by mid-17c. it had acquired a meaning of "roguish, mischievous," softened by 19c. to "saucy." The shifting sense is exemplified by archwife (late 14c.), variously defined as "a wife of a superior order" or "a dominating woman, virago." Related: Archly; archness.
arch-
also archi-, word-forming element meaning "chief, principal; extreme, ultra; early, primitive," from Latinized form of Greek arkh-, arkhi- "first, chief, primeval," comb. form of arkhos "a chief, leader, commander," arkhein "be first, begin" (see archon).
arch-enemy (n.)
also archenemy, 1540s, from arch- + enemy. Originally especially Satan.
arch-fiend (n.)
1667, from arch (adj.) + fiend (n.). Originally and typically Satan (arch-foe "Satan" is from 1610s).
So stretcht out huge in length the Arch-fiend lay. ["Paradise Lost," 1667]
arch-rival (n.)
also archrival, by 1805, from arch- + rival (n.).
arch-villain (n.)
c. 1600, from arch- + villain.
Archaean (adj.)
"of the earliest geological age," 1872, coined by U.S. geologist James Dwight Dana (1813-1895) from Latinized form of Greek arkhaios "ancient," from arkhe "beginning" (see archon).
archaebacteria (n.)
1977, from archaeo- "ancient" + bacteria. Singular is archaebacterium.
archaeo-
before vowels archae-, word-forming element in scientific compounds meaning "ancient, olden, primitive, primeval, from the beginning," from Latinized form of Greek arkhaios "ancient, primeval," from arkhe "beginning" (see archon).
archaeoastronomy (n.)
1971, from archaeo- "ancient" + astronomy.
archaeological (adj.)
"pertaining to archaeology," 1766, in the antiquarian sense, from archaeology + -ical. Earlier was archaeologic (1731). Related: Archaeologically.
archaeologist (n.)
1824; see archaeology + -ist. Other early forms were archaeologian (1820), archaeologue (1839, from French archéologue). Greek arkhaiologos meant "antiquary."
archaeology (n.)
c. 1600, "ancient history," from French archéologie (16c.) or directly from Greek arkhaiologia "the study of ancient things;" see archaeo- + -ology. Meaning "scientific study of ancient peoples and past civilizations" is recorded by 1825.
archaeopteryx (n.)
Jurassic fossil animal long considered the oldest known bird (in 21c. new candidates emerged), 1871, from German (1861), coined in Modern Latin by German paleontologist Christian Erich Hermann von Meyer (1801-1869), from archaeo- "ancient, primitive" + Greek pteryx "wing" (see pterodactyl). Discovered (first as a single feather) by Andreas Wagner in 1860 or '61 in Bavaria.
archaic (adj.)
1810, from or by influence of French archaique (1776), ultimately from Greek arkhaikos "old-fashioned," from arkhaios "ancient, old-fashioned, antiquated, primitive," from arkhe "beginning" (see archon). Not merely crude, the archaic has "a rudeness and imperfection implying the promise of future advance" [Century Dictionary]. Archaical is attested from 1799.
archaism (n.)
1640s, "retention of what is old and obsolete," from Modern Latin archaismus, from Greek arkhaismos, from arkhaizein "to copy the ancients" (in language, etc.); see archaic. Meaning "that which is archaic," especially "an archaic word or expression," is by 1748.