Online Etymology Dictionary

armchair (n.): also arm-chair, 1630s, from arm (n.1) + chair; adjective sense in reference to "criticism of matters in which the critic takes no active part" is from 1886.
armed (adj.): "equipped for battle," early 13c., past participle adjective from arm (v.).
Armenian: 1590s, "a native of Armenia," from Armenia (late 14c. in English), place name traced to 521 C.E., but of uncertain origin. As the name of the language, by 1718; as an adjective, by 1727.
armilla (n.): 1706, Latin, literally "bracelet, armlet, arm ring," from armus "shoulder, upper arm" (see arm (n.1)). Related: Armillary.
Arminian: 1610s, from Arminius, Latinized form of the name of James Harmensen (1560-1609), Dutch Protestant theologian who opposed Calvin, especially on the question of predestination. His ideas were denounced at the Synod of Dort, but nonetheless spread in the Reformed churches.
armistice (n.): 1707, from French armistice (1680s), coined on the model of Latin solstitium (see solstice), etc., from Latin arma "arms" (see arm (n.2)) + -stitium (used only in compounds), from sistere "cause to stand" (see assist).

The word is attested in English from 1660s in the Latin form armistitium. German Waffenstillstand is a loan-translation from French. Armistice Day (1919) marked the end of the Great War of 1914-18 on Nov. 11, 1918. In Britain, after World War II, it merged with Remembrance Day. In U.S., Armistice Day became a national holiday in 1926. In 1954, to honor World War II and Korean War veterans as well, it was re-dubbed Veterans Day.
armless (adj.): late 14c., of physical conditions, from arm (n.1) + -less. Meaning "without weapons" is attested from 1610s (from arm (n.2)), but that sense is more typically expressed by unarmed.
armlet (n.): 1530s, diminutive of arm (n.1).
armoire (n.): 1570s, from French armoire, from Old French armarie (12c.) "cupboard, bookcase, reliquary," from Latin armarium "closet, chest, place for implements or tools," from arma "gear, tools, arms" (see arm (n.2)). Before being reborrowed from French, the word earlier was in English as ambry (late 14c.).
armor (n.): c.1300, "mail, defensive covering worn in combat," also "means of protection," from Old French armeure "weapons, armor" (12c.), from Latin armatura "arms, equipment," from arma "arms, gear" (see arm (n.2)). Figurative use from mid-14c.

Meaning "military equipment generally," especially siege engines, is late 14c. The word might have died with jousting if not for late 19c. transference to metal-shielded machinery beginning with U.S. Civil War ironclads (first attested in this sense in an 1855 report from the U.S. Congressional Committee on Naval Affairs).
armor (v.): mid-15c., from armor (n.). Related: Armored; armoring.
armorer (n.): late 14c. (mid-13c. as a surname), from Anglo-French armurer, Old French aremurier, from armeure "armor" (see armor (n.)).
armorial (adj.): 1570s, from armory + -al (1).
Armorica: ancient name for Brittany, from Gallo-Romance Are-mor-ica, literally "before the sea," with a Celtic prefix meaning "before" (cf. Old Irish ar) + mare "sea" (see mere (n.)).
armory (n.): "arms and weapons collectively," c.1300; see arm (n.2) + -ory. Meaning "place where arms are manufactured" is from mid-15c. Also used in a sense of "arsenal" (mid-15c.), "the science of heraldry" (late 15c.), from Old French armoierie, from armoier "to blazon," from Latin arma "weapons" (see arm (n.2)).
armour: chiefly British English spelling of armor (q.v.); for suffix, see -or.
armoury (n.): chiefly British English spelling of armory (q.v.); for suffix, see -or.
armpit (n.): mid-14c., from arm (n.1) + pit (n.1). Arm-hole (early 14c.) was used in this sense but was obsolete by 18c. Colloquial armpit of the nation for any locale regarded as ugly and disgusting was in use by 1965.
arms race (n.): 1930, in reference to naval build-ups, from arms (see arm (n.2)) + race (n.1). First used in British English.
arms-length (n.): 1650s, from arm (n.1) + length. At arm's end is recorded from 1570s.
army (n.): late 14c., "armed expedition," from Old French armée (14c.) "armed troop, armed expedition," from Medieval Latin armata "armed force," from Latin armata, fem. of armatus "armed, equipped, in arms," past participle of armare "to arm," literally "act of arming," related to arma "tools, arms" (see arm (n.2)). Originally used of expeditions on sea or land; the specific meaning "land force" first recorded 1786. Transferred meaning "host, multitude" is c.1500.

The Old English words were here (still preserved in derivatives like harrier), from PIE *kor- "people, crowd;" and fierd, with an original sense of "expedition," from faran "travel." In spite of etymology, in the Anglo-Saxon Chronicle, here generally meant "invading Vikings" and fierd was used for the local militias raised to fight them.
arnica (n.): plant genus of the borage family, 1753, Modern Latin, of unknown origin. Klein suggests Arabic arnabiyah, a name of a type of plant, as the ultimate source.
Arnold: masc. proper name, from Old High German Arenwald, literally "having the strength of an eagle," from arn "eagle" (see erne) + wald "power" (see wield).
aroint (v.): intransitive verb, c.1600, used by Shakespeare (only in imperative: "begone!"), obsolete and of obscure origin. "[T]he subject of numerous conjectures, none of which can be said to have even a prima facie probability." [OED]
aroma (n.): early 13c., "fragrant substance," from Latin aroma "sweet odor," from Greek aroma "seasoning, any spice or sweet herb," of unknown origin. Meaning "fragrance" is from 1814. A hypercorrect plural is aromata.
aromatherapy (n.): by 1992, from French aromathérapie, attested from 1930s; see aroma + therapy.
aromatic (adj.): c.1400, aromatyk, from Middle French aromatique (14c.), from Latin aromaticus, from Greek aromatikos, from aroma (genitive aromatos) "seasoning, sweet spice," of unknown origin.
aromatize (v.): late 15c., from Old French aromatiser (12c.), from Latin aromatizare, from Greek aromatizein "to spice," from aromat-, stem of aroma "seasoning, sweet spice" (see aroma).
arose (v.): past tense of arise (v.).
around (adv.): c.1300, "in circumference," from phrase on round. Rare before 1600. In sense of "here and there with no fixed direction" it is 1776, American English (properly about). Of time, from 1888. To have been around "gained worldly experience" is from 1927, U.S. colloquial.
arousal (n.): 1827, "action of arousing, a being awakened" from arouse + -al (2). Sexual association is from c.1900.
arouse (v.): 1590s, "awaken" (transitive), from a- (1) "on" + rouse. Related: Aroused; arousing.
ARPANET: acronym from Advanced Research Projects Agency Network, set up in 1969 by a branch of the U.S. Department of Defense in partnership with four universities; acknowledged as "the world's first operational packet switching network" and predecessor of the Internet.
arpeggio (n.): 1742, from Italian arpeggio, from arpeggiare "to play upon the harp," from arpa "harp," which is of Germanic origin (see harp (n.)). Related: Arpeggiated; arpeggiation.
arr (v.): "to growl like a dog," late 15c., imitative. In classical times, the letter R was called littera canina "the dog letter" (Persius).
arrack (n.): c.1600, probably picked up in India, ultimately from Arabic araq, literally "sweat, juice;" used of native liquors in Eastern countries, especially those distilled from fermented sap of coconut palm, sometimes from rice or molasses.
arrah: supposedly a characteristic Irish expression of emotion or excitement, 1705.
arraign (v.): late 14c., araynen, "to call to account," from Old French araisnier "speak to, address; accuse (in a law court)," from Vulgar Latin *arrationare, from Latin adrationare, from ad- "to" (see ad-) + *rationare, from ratio "argumentation, reckoning, calculation" (see ratio). Sense of "to call up on a criminal charge" is c.1400. The excrescent -g- is a 16c. overcorrection based on reign, etc. Related: Arraigned; arraigning.
arraignment (n.): mid-15c., from Old French araisnement, from araisnier (see arraign).
arrange (v.): late 14c., "draw up a line of battle," from Old French arengier (12c.), from a- "to" (see ad-) + rangier "set in a row" (Modern French ranger), from rang "rank," from Frankish *hring (see rank (n.)).

A rare word until the meaning generalized to "to place things in order" c.1780-1800. Musical sense of "adapt for other instruments or voices" is from 1808. Related: Arranged; arranging. Arranged marriage attested from 1854.
arrangement (n.): 1727, from French arrangement, from arranger "arrange" (see arrange).
arrant (adj.): late 14c., variant of errant (q.v.); at first merely derogatory, "wandering, vagrant;" then (1540s) acquiring a meaning "thoroughgoing, downright, notorious."
arras (n.): "pictured tapestry," late 14c., from Anglo-French draps d'arras, from Arras, city in France where pictured tapestries were made, from Latin Atrebates, name of a tribe of the Belgae who inhabited the Artois region; probably literally "inhabitants," from a Celtic trebu "tribe."
array (v.): early 14c., from stem of Old French areer "to put in order," from Vulgar Latin *ar-redare (source of Italian arredare), from Latin ad- "to" (see ad-) + Frankish *ræd- "ready" or some cognate Germanic source, from Proto-Germanic *raidjan "to place in order" (cf. Gothic garadis, Old English geræde "ready;" see ready). Related: Arrayed; arraying.
array (n.): mid-14c., "order, arrangement," from Anglo-French arrai, Old French aroi, from areer (see array (v.)).
arrear: c.1300, "at a disadvantage;" mid-14c., "in times past;" late 14c., "in or to the rear;" see arrears. Meaning "behind in duties or payments" is from 1620s.
arrearage (n.): "unpaid debt," early 14c., from Old French arierage "detriment, prejudice (in a legal sense)" (Modern French arrérage), from arere "behind;" see arrears.
arrears (n.): mid-14c., "in times past," from Old French ariere "behind, backward," from Vulgar Latin *ad retro, from Latin ad "to" (see ad-) + retro "behind" (see retro-). Meaning "balance due" dates from early 15c.; phrase in arrears first recorded 1610s, but in arrearages is from late 14c.
arrest (v.): "to cause to stop," also "to detain legally," late 14c., from Old French arester "to stay, stop" (Modern French arrêter), from Vulgar Latin *arrestare (source of Italian arrestare, Spanish and Portuguese arrestar), from Latin ad- "to" (see ad-) + restare "to stop, remain behind, stay back" (see rest (n.2)). Figurative sense of "to catch and hold" (the attention, etc.) is from 1814.
arrest (n.): late 14c., from Anglo-French arest, Old French areste, from arester (see arrest (v.)).