Online Etymology Dictionary

arational (adj.): "not purporting to be governed by laws of reason," 1935; see a- (2) + rational.
arbalest (n.): "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" (see ballistic). German armbrust is from the same French word but mangled by folk etymology.
arbiter (n.): late 14c., from Old French arbitre or directly from Latin arbiter "one who goes somewhere (as witness or judge)," in classical Latin used of spectators and eye-witnesses, 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. The earliest form of the word attested in English is the fem. noun arbitress (mid-14c.) "a woman who settles disputes."
arbitrage (n.): "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 (see arbiter).
arbitrary (adj.): early 15c., "deciding by one's own discretion," from Old French arbitraire (14c.) or directly from Latin arbitrarius "depending on the will, uncertain," from arbiter (see arbiter). The original meaning gradually descended to "capricious" and "despotic" (1640s). Related: Arbitrarily; arbitrariness.
arbitrate (v.): 1580s (arbitrable is recorded from 1530s), "to give an authoritative decision," from Latin arbitratus, past participle of arbitrari "be of an opinion, give a decision," from arbiter (see arbiter). Meaning "to act as an arbitrator" is from 1610s. Related: Arbitrated; arbitrating. The earlier verb form was arbitren (early 15c.).
arbitration (n.): late 14c., "absolute decision," from Old French arbitracion, 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 (see arbiter). Meaning "settlement of a dispute by a third party" is from 1630s.
arbitrator (n.): early 15c., from Old French arbitratour (13c.), from Latin arbitrator "a spectator, hearer, witness, judge," agent noun from past participle stem of arbitrari, from arbiter (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 (early 15c.) decides issues referred to him by the parties.
arbitrer (n.): late 14c., from Anglo-French arbitrour, Old French arbitreor (13c.), from Old French arbitrer (see arbitrage).
arbor (n.): c.1300, herber, "herb garden," from Old French erbier "field, meadow; kitchen garden," from Latin herba "grass, herb" (see herb). Later "a grassy plot" (early 14c., a sense also in Old French), "a shaded nook" (mid-14c.). Probably not from Latin arbor "tree," though perhaps influenced by its spelling.

The change from er- to ar- before consonants in Middle English also reflects a pronunciation shift: cf. farm from ferme, harbor from Old English herebeorg.
Arbor Day: the day set aside for the planting of trees, first celebrated 1872 in Nebraska, the brainchild of U.S. agriculturalist and journalist J. Sterling Morton (1832-1902). From Latin arbor "tree," of unknown origin.
arbor vitae (n.): type of evergreen shrub, 1660s, name given by French physician and botanist Charles de Lécluse (1525-1609), Latin, literally "tree of life." Also used in late 18c. rogue's slang as a cant word for "penis."
arboreal (adj.): 1660s, from Latin arboreus "pertaining to trees," from arbor "tree," of unknown origin, + -al (1).
arboretum (n.): "tree-garden," 1838, from Latin arboretum, literally "a place grown with trees," from arbor "tree," of unknown origin, + -etum, suffix used to form the names of gardens and woods.
arborist (n.): 1570s, from Latin arbor "tree," of unknown origin, + -ist. In early use probably from French arboriste.
arbour (n.): chiefly British English spelling of arbor (q.v.); for spelling, see -or.
arc (n.): late 14c., originally in reference to the sun's apparent motion in the sky, from Old French arc "bow, arch, vault" (12c.), from Latin arcus "a bow, arch," from PIE root *arku- "bowed, curved" (cf. Gothic arhvazna "arrow," Old English earh, Old Norse ör; also, via notion of "supple, flexible," Greek arkeuthos, Latvian ercis "juniper," Russian rakita, Czech rokyta, Serbo-Croatian rakita "brittle willow"). Electrical sense is from 1821.
arc (v.): 1893, in the electrical sense, from arc (n.). Meaning "to move in an arc" attested by 1954. Related: Arced; arcing.
arcade (n.): 1731 (as arcado, from 1640s), from Italian arcata "arch of a bridge," from arco "arc," from Latin arcus (see arc). Applied to passages formed by a succession of arches, avenues of trees, and ultimately to any covered avenue, especially one lined with shops (1731) or amusements; hence arcade game (1977).
Arcadia: see Arcadian.
Arcadian: "ideally rustic or rural; an idealized rustic," 1580s, from Greek Arkadia, district in the Peloponnesus, taken by poets as an ideal region of rural felicity, traditionally from Arkas (genitive Arkadas), son of Zeus, name of the founder and first ruler of Arcadia.
arcana (n.): "hidden things, mysteries," 1590s, a direct adoption of the Latin plural of arcanum "a secret, a mystery," 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 "close up, enclose, contain," from arca "chest, box, place for safe-keeping," from PIE *ark- "to hold, contain, guard" (cf. Greek arkos "defense," arkein "to ward off;" Armenian argel "obstacle;" Lithuanian raktas "key," rakinti "to shut, lock").
arcanum (n.): proper singular form of arcana.
arch (n.): c.1300, from Old French arche "arch of a bridge" (12c.), from Latin arcus "a bow" (see arc). Replaced native bow (n.1). Originally architectural in English; transferred by early 15c. to anything having this form (eyebrows, etc.).
arch (adj.): 1540s, "chief, principal," from prefix arch-; used in 12c. archangel, etc., but extended to so many derogatory uses (arch-rogue, arch-knave, etc.) that by mid-17c. it acquired a meaning of "roguish, mischievous," since softened to "saucy." Also found in archwife (late 14c.), variously defined as "a wife of a superior order" or "a dominating woman, virago."
arch (v.): early 14c., "to form an arch" (implied in arched); c.1400, "to furnish with an arch," from arch (n.). Related: Arching.
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 "chief" (see archon).
Archaean (adj.): "of the earliest geological age," 1872, from Greek arkhaios "ancient," from arkhe "beginning" (see archon).
archaebacteria (n.): plural of archaebacterium (1977), from archaeo- + bacterium (see bacteria).
archaeo-: before vowels archae-, word-forming element meaning "ancient, olden, primitive, primeval, from the beginning," from Latinized form of Greek arkhaio-, comb. form of arkhaios "ancient," from arkhe "beginning" (see archon).
archaeoastronomy (n.): 1971, from archaeo- + astronomy.
archaeologist (n.): 1824; see archaeology + -ist.
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" recorded by 1825. Related: Archaeological; archaeologically.
archaeopteryx (n.): oldest known fossil bird, 1859, Modern Latin, from archaeo- "ancient, primitive" + Greek pteryx "wing" (see pterodactyl).
archaic (adj.): 1810, from or by influence of French archaique (1776), ultimately from Greek arkhaikos "old-fashioned," from arkhaios "ancient," from arkhe "beginning" (see archon). 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 "an archaic word or expression" is from c.1748.
archangel (n.): late 12c., from Old French archangel (12c.) or directly from Late Latin archangelus, from Greek arkhangelos "chief angel," from arkh- "chief, first" (see archon) + angelos (see angel). Replaced Old English heah encgel.
archangelic (adj.): mid-15c.; see archangel + -ic.
archbishop (n.): Old English ærcebiscop, from Late Latin archiepiscopus, from Greek arkhi- "chief" (see archon) + episkopos "bishop," literally "overseer." Replaced earlier Old English heah biscop (see bishop). The spelling conformed to Latin from 12c.
archbishopric (n.): Old English arcebiscoprice, from archbishop + rice "realm, dominion, province" (see regal).
archdeacon (n.): Old English arcediacon, from Church Latin archidiaconus, from Ecclesiastical Greek arkhidiakonon "chief deacon;" see arch- + deacon.
archdiocese (n.): 1762, from arch- + diocese.
archduchess (n.): 1610s; see arch- + duchess. Also cf. archduke.
archduke (n.): 1520s, from Middle French and Old French archeduc, from Merovingian Latin archiducem (c.750); see arch- + duke (n.). Formerly the title of the rulers of Austrasia, Lorraine, Brabant, and Austria; later the titular dignity of the sons of the Emperor of Austria. Related: Archducal; archduchy.
archenemy (n.): also arch-enemy, 1540s, from arch- + enemy.
archeological (adj.): alternative spelling of archaeological (see archaeology). Also see ae.
archeologist (n.): alternative spelling of archaeologist. Also see ae.
archeology (n.): alternative spelling of archaeology. Also see ae.
archer (n.): late 13c., from Anglo-French archer, Old French archier "archer, bowmaker," from Latin arcarius, from arcus "bow" (see arc). Also a 17c. name for the bishop in chess.