Armageddon (n.) Look up Armageddon at Dictionary.com
"a final conflict," 1811, figurative use of name in Rev. xvi:16, place of the great and final conflict, from Hebrew Har Megiddon "Mount of Megiddo," city in central Palestine, site of important Israeli battles.
armament (n.) Look up armament at Dictionary.com
c.1600, "munitions of war" (especially the great guns on board a man-of-war), also "naval force equipped for war" (1690s), from Latin armamentum "implement," from Latin armare "to arm, furnish with weapons" from arma (see arm (n.2)). Meaning "process of equipping for war" is from 1813.
armamentarium (n.) Look up armamentarium at Dictionary.com
1874, Latin, literally "little arsenal," from armamentum (see armament). Englished as armamentary (1731).
armature (n.) Look up armature at Dictionary.com
c.1400, "an armed force," from Latin armatura "armor, equipment," from armatus, past participle of armare "to arm, furnish with weapons" from arma (see arm (n.2)). Meaning "armor" is mid-15c.; that of "protective covering of a plant or animal" is from 1660s. Electromagnetic sense is from 1835.
armchair (n.) Look up armchair at Dictionary.com
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. Another old name for it was elbow-chair (1650s).
armed (adj.) Look up armed at Dictionary.com
"equipped for battle," early 13c., past participle adjective from arm (v.).
Armenian Look up Armenian at Dictionary.com
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.) Look up armilla at Dictionary.com
1706, Latin, literally "bracelet, armlet, arm ring," from armus "shoulder, upper arm" (see arm (n.1)). Related: Armillary.
Arminian Look up Arminian at Dictionary.com
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.) Look up armistice at Dictionary.com
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 PIE *ste-ti-, suffixed form of root *sta- "to stand" (see stet).

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.) Look up armless at Dictionary.com
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.) Look up armlet at Dictionary.com
1530s, diminutive of arm (n.1).
armoire (n.) Look up armoire at Dictionary.com
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.) Look up armor at Dictionary.com
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.) Look up armor at Dictionary.com
mid-15c., from armor (n.). Related: Armored; armoring.
armorer (n.) Look up armorer at Dictionary.com
late 14c. (mid-13c. as a surname), from Anglo-French armurer, Old French aremurier, from armeure "armor" (see armor (n.)).
armorial (adj.) Look up armorial at Dictionary.com
1570s, from armory + -al (1).
Armorica Look up Armorica at Dictionary.com
ancient name for Brittany, from Gallo-Roman Are-mor-ica, literally "before the sea," with a Celtic prefix meaning "before" (compare Old Irish ar) + mare "sea" (see mere (n.)).
armory (n.) Look up armory at Dictionary.com
"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 Look up armour at Dictionary.com
chiefly British English spelling of armor (q.v.); for suffix, see -or.
armoury (n.) Look up armoury at Dictionary.com
chiefly British English spelling of armory (q.v.); for suffix, see -or.
armpit (n.) Look up armpit at Dictionary.com
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.) Look up arms race at Dictionary.com
1930, in reference to naval build-ups, from arms (see arm (n.2)) + race (n.1). First used in British English.
arms-length (n.) Look up arms-length at Dictionary.com
1650s, from arm (n.1) + length. At arm's end is recorded from 1570s.
army (n.) Look up army at Dictionary.com
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.) Look up arnica at Dictionary.com
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 Look up Arnold at Dictionary.com
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.) Look up aroint at Dictionary.com
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.) Look up aroma at Dictionary.com
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.) Look up aromatherapy at Dictionary.com
by 1992, from French aromathérapie, attested from 1930s; see aroma + therapy.
aromatic (adj.) Look up aromatic at Dictionary.com
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.) Look up aromatize at Dictionary.com
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.) Look up arose at Dictionary.com
past tense of arise (v.).
around (adv.) Look up around at Dictionary.com
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.) Look up arousal at Dictionary.com
1827, "action of arousing, a being awakened" from arouse + -al (2). Sexual association is from c.1900.
arouse (v.) Look up arouse at Dictionary.com
1590s, "awaken" (transitive), from a- (1) "on" + rouse. Related: Aroused; arousing.
ARPANET Look up ARPANET at Dictionary.com
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.) Look up arpeggio at Dictionary.com
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.) Look up arr at Dictionary.com
"to growl like a dog," late 15c., imitative. In classical times, the letter R was called littera canina "the dog letter" (Persius).
arrack (n.) Look up arrack at Dictionary.com
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 Look up arrah at Dictionary.com
supposedly a characteristic Irish expression of emotion or excitement, 1705.
arraign (v.) Look up arraign at Dictionary.com
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.) Look up arraignment at Dictionary.com
mid-15c., from Old French araisnement, from araisnier (see arraign).
arrange (v.) Look up arrange at Dictionary.com
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.) Look up arrangement at Dictionary.com
1727, from French arrangement, from arranger "arrange" (see arrange).
arrant (adj.) Look up arrant at Dictionary.com
late 14c., variant of errant (q.v.); at first merely derogatory, "wandering, vagrant;" then (1540s) acquiring a meaning "thoroughgoing, downright, notorious."
arras (n.) Look up arras at Dictionary.com
"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.) Look up array at Dictionary.com
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" (cognates: Gothic garadis, Old English geræde "ready;" see ready (adj.)). Related: Arrayed; arraying.
array (n.) Look up array at Dictionary.com
mid-14c., "order, arrangement," from Anglo-French arrai, Old French aroi, from areer (see array (v.)).
arrear Look up arrear at Dictionary.com
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.