Online Etymology Dictionary

aquarium (n.): 1830, noun use of neuter of Latin aquarius "pertaining to water," as a noun, "water-carrier," genitive of aqua "water" (see aqua-). The word existed in Latin, but there it meant "drinking place for cattle." Originally especially for growing aquatic plants; An earlier attempt at a name for "fish tank" was marine vivarium.
Aquarius: faint constellation and 11th zodiac sign, late Old English, from Latin aquarius, literally "water carrier," properly an adjective, "pertaining to water" (see aquarium); a loan-translation of Greek Hydrokhoos "the water-pourer," old Greek name of this constellation.

Aquarians were a former Christian sect that used water instead of wine at the Lord's Supper. Age of Aquarius (1940) is an astrological epoch supposed to have begun in the 1960s, embodying the traits of this sign and characterized by world peace and human brotherhood. The term and the concept probably got a boost in popular use when An Aquarian Exposition was used as the sub-name of the Woodstock Music & Art Fair (1969).
aquatic (adj.): late 15c., from Middle French and Old French aquatique (13c.), from Latin aquaticus "growing in water; bringing rain," from aqua "water" (see aqua-)
aquatint (n.): 1782, "engraving made with aqua fortis," from Italian acquatinta, from Latin aqua tincta "dyed water;" see aqua- + tinct.
aqueduct (n.): 1530s, from Latin aquaeductus "conveyance of water," from aquae, genitive of aqua "water" (see aqua-), + ductus "a leading, conducting," past participle of ducere "to lead" (see duke (n.)).
aqueous (adj.): 1640s, from Latin aqua "water" (see aqua-) on analogy of French aqueux "watery" (16c., which, however, is from Late Latin aquosus "abounding in water"). Or by analogy of Latin terreus "earthy," from terra "earth." Aqueous humor is the original use in English.
aquifer (n.): 1897, coined from Latin aqui-, comb. form of aqua "water" (see aqua-) + -fer "bearing," from ferre "to bear, carry" (see infer).
aquiline (adj.): "curved like an eagle's beak," 1640s, originally in English in reference to long, hooked noses, from Latin aquilinus "of or like an eagle," from aquila "eagle," of uncertain origin, usually explained as "the dark bird;" compare aquilus "blackish, of the color of darkness."
aquiver (adv.): 1864, from a- (1) + quiver (v.).
Arab (n.): late 14c. (Arabes, a plural form), from Old French Arabi, from Latin Arabs (accusative Arabem), from Greek Araps (genitive Arabos), from Arabic 'arab, indigenous name of the people, perhaps literally "inhabitant of the desert" and related to Hebrew arabha "desert." Meaning "homeless little wanderer, child of the street" is from 1848 (originally Arab of the city), in reference to nomadic ways. Arab League formed in Cairo, March 22, 1945.
arabesque (n.): 1610s, "Moorish or Arabic ornamental design," from French arabesque (16c.), from Italian arabesco, from Arabo "Arab," with reference to Moorish architecture. As a ballet pose, first attested 1830. Musical sense, in reference to an ornamented theme, is from 1864, originally the title given by Robert Schumann to one of his piano pieces.
Arabia: 1711; see Arab + -ia. The older name for "the country of Arabia" was Araby (late 13c.).
Arabian: c. 1300, adjective and noun; see Arab + -ian. As a prized type of horse, it is attested from 1660s. The Arabian bird was the phoenix.
Arabic (adj.): early 14c., from Old French Arabique (13c.), from Latin Arabicus "Arabic" (see Arab). Old English used Arabisc "Arabish." Originally in reference to gum arabic; noun meaning "Arabic language" is from late 14c.

Arabic numerals (actually Indian) first attested 1727; they were introduced in Europe by Gerbert of Aurillac (later Pope Sylvester II) after a visit to Islamic Spain in 967-970. A prominent man of science, he taught in the diocesan school at Reims, but the numbers made little headway against conservative opposition in the Church until after the Crusades. The earliest depiction of them in English, in "The Crafte of Nombrynge" (c. 1350) correctly identifies them as "teen figurys of Inde."
arable (adj.): early 15c., "suitable for plowing" (as opposed to pasture- or wood-land), from Old French arable (12c.), from Latin arabilis, from arare "to plow," from PIE *are- "to plow" (source also of Greek aroun, Old Church Slavonic orja, Lithuanian ariu "to plow;" Gothic arjan, Old English erian, Middle Irish airim, Welsh arddu "to plow;" Old Norse arþr "a plow"). By late 18c. it replaced native erable, from Old English erian "to plow," from the same PIE source.
arachnid (n.): "a spider," 1869, from French arachnide (1806) or Modern Latin Arachnida, introduced as name for this class of arthropods 1815 by French biologist Jean Baptiste Pierre Antoine de Monet de Lamarck (1744-1829), from Greek arakhne (fem.) "spider; spider's web," which probably is cognate with Latin aranea "spider, spider's web" (borrowed in Old English as renge "spider"), from aracsna. The Latin word could be a Greek borrowing or both could be from a common root. An earlier noun form was arachnidian (1828). The Latin word appears in English as arain, noted in John Ray's "Collection of English Words" (1768) as a Nottinghamshire word for "the larger kind of spiders."
arachnoid (adj.): "cobweb-like," especially of the membrane around the brain and spinal cord, 1789, from Modern Latin arachnoides, from Greek arakhnoeides "cobweb-like," from arakhne "cobweb" (see arachnid) + -oeides (see -oid).
arachnologist (n.): student of spiders, 1806; see arachnid + -ology. Related: Arachnology.
arachnophobia (n.): 1925, from comb. form of arachnid + -phobia "fear."
Aragon: medieval Spanish kingdom, named for a river that runs through it, probably from a PIE root meaning "water."
Aramaic: 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.): "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: compare 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," which is 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," which is of unknown origin, + -al (1).
arboretum (n.): "tree-garden," 1838, from Latin arboretum, literally "a place grown with trees," from arbor "tree," which is of unknown origin, + -etum, suffix used to form the names of gardens and woods.
arborist (n.): 1570s, from Latin arbor "tree," which is 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" (source also of 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" (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.
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).