arachnologist (n.) Look up arachnologist at Dictionary.com
student of spiders, 1806; see arachnid + -ology. Related: Arachnology.
arachnophobia (n.) Look up arachnophobia at Dictionary.com
1925, from comb. form of arachnid + -phobia "fear."
Aragon Look up Aragon at Dictionary.com
medieval Spanish kingdom, named for a river that runs through it, probably from a PIE root meaning "water."
Aramaic Look up Aramaic at Dictionary.com
northern branch of Semitic language group, 1834, from biblical land of Aram, roughly corresponding to modern Syria; probably related to Hebrew and Aramaic rum "to be high," thus originally "highland." Aramaic became the lingua franca of the Assyrian empire and later for centuries was the official language of the Persian kingdom and the daily language of Israel at the time of Christ.
arational (adj.) Look up arational at Dictionary.com
"not purporting to be governed by laws of reason," 1935; see a- (2) + rational.
arbalest (n.) Look up arbalest at Dictionary.com
"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.) Look up arbiter at Dictionary.com
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.) Look up arbitrage at Dictionary.com
"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.) Look up arbitrary at Dictionary.com
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.) Look up arbitrate at Dictionary.com
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.) Look up arbitration at Dictionary.com
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.) Look up arbitrator at Dictionary.com
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.) Look up arbitrer at Dictionary.com
late 14c., from Anglo-French arbitrour, Old French arbitreor (13c.), from Old French arbitrer (see arbitrage).
arbor (n.) Look up arbor at Dictionary.com
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: compare farm from ferme, harbor from Old English herebeorg.
Arbor Day Look up Arbor Day at Dictionary.com
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.) Look up arbor vitae at Dictionary.com
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.) Look up arboreal at Dictionary.com
1660s, from Latin arboreus "pertaining to trees," from arbor "tree," of unknown origin, + -al (1).
arboretum (n.) Look up arboretum at Dictionary.com
"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.) Look up arborist at Dictionary.com
1570s, from Latin arbor "tree," of unknown origin, + -ist. In early use probably from French arboriste.
arbour (n.) Look up arbour at Dictionary.com
chiefly British English spelling of arbor (q.v.); for spelling, see -or.
arc (n.) Look up arc at Dictionary.com
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" (cognates: 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.) Look up arc at Dictionary.com
1893, in the electrical sense, from arc (n.). Meaning "to move in an arc" attested by 1954. Related: Arced; arcing.
arcade (n.) Look up arcade at Dictionary.com
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 Look up Arcadia at Dictionary.com
see Arcadian.
Arcadian Look up Arcadian at Dictionary.com
"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.) Look up arcana at Dictionary.com
"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.) Look up arcane at Dictionary.com
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" (cognates: Greek arkos "defense," arkein "to ward off;" Armenian argel "obstacle;" Lithuanian raktas "key," rakinti "to shut, lock").
arcanum (n.) Look up arcanum at Dictionary.com
proper singular form of arcana.
arch (n.) Look up arch at Dictionary.com
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.) Look up arch at Dictionary.com
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.) Look up arch at Dictionary.com
early 14c., "to form an arch" (implied in arched); c.1400, "to furnish with an arch," from arch (n.). Related: Arching.
arch- Look up arch- at Dictionary.com
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.) Look up Archaean at Dictionary.com
"of the earliest geological age," 1872, from Greek arkhaios "ancient," from arkhe "beginning" (see archon).
archaebacteria (n.) Look up archaebacteria at Dictionary.com
plural of archaebacterium (1977), from archaeo- + bacterium (see bacteria).
archaeo- Look up archaeo- at Dictionary.com
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.) Look up archaeoastronomy at Dictionary.com
1971, from archaeo- + astronomy.
archaeologist (n.) Look up archaeologist at Dictionary.com
1824; see archaeology + -ist.
archaeology (n.) Look up archaeology at Dictionary.com
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.) Look up archaeopteryx at Dictionary.com
oldest known fossil bird, 1859, Modern Latin, from archaeo- "ancient, primitive" + Greek pteryx "wing" (see pterodactyl).
archaic (adj.) Look up archaic at Dictionary.com
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.) Look up archaism at Dictionary.com
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.) Look up archangel at Dictionary.com
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.) Look up archangelic at Dictionary.com
mid-15c.; see archangel + -ic.
archbishop (n.) Look up archbishop at Dictionary.com
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.) Look up archbishopric at Dictionary.com
Old English arcebiscoprice, from archbishop + rice "realm, dominion, province" (see regal).
archdeacon (n.) Look up archdeacon at Dictionary.com
Old English arcediacon, from Church Latin archidiaconus, from Ecclesiastical Greek arkhidiakonon "chief deacon;" see arch- + deacon.
archdiocese (n.) Look up archdiocese at Dictionary.com
1762, from arch- + diocese.
archduchess (n.) Look up archduchess at Dictionary.com
1610s; see arch- + duchess. Also compare archduke.
archduke (n.) Look up archduke at Dictionary.com
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.) Look up archenemy at Dictionary.com
also arch-enemy, 1540s, from arch- + enemy.