arithmetic (n.) Look up arithmetic at Dictionary.com
"art of computation, the most elementary branch of mathematics," mid-13c., arsmetike, from Old French arsmetique (12c.), from Latin arithmetica, from Greek arithmetike (tekhne) "(the) counting (art)," fem. of arithmetikos "of or for reckoning, arithmetical," from arithmos "number, counting, amount," from PIE root *re(i)- "to reason, count" (source also of Old English, Old High German rim "number;" Old Irish rim "number," dorimu "I count;" Latin ritus "religious custom;" see read).

The form arsmetrik was based on folk-etymology derivation from Medieval Latin ars metrica; the spelling was corrected early 16c. in English (though arsmetry is attested from 1590s) and French. The native formation in Old English was tælcræft, literally "tell-craft."
arithmetical (adj.) Look up arithmetical at Dictionary.com
"pertaining to or according to the rules of arithemetic," 1540s; see arithmetic + -al (1). In modern use, opposed to geometrical. Related: Arithmetically (late 15c.).
arithmocracy (n.) Look up arithmocracy at Dictionary.com
"rule by numerical majority," 1850, from Greek arithmos "number, counting, amount" (see arithmetic) + -cracy. Related: Arithmocratic; arithmocratical.
arithmomania (n.) Look up arithmomania at Dictionary.com
"compulsive desire to count objects and make calculations," 1884, from French arithmomanie, from Greek arithmos "number, counting, amount" (see arithmetic) + French -manie (see mania). Related: Arithmomaniac.
Arizona Look up Arizona at Dictionary.com
1861, originally as the name of a breakaway Confederate region of southern New Mexico; organized roughly along modern lines as a U.S. territory in 1863, admitted as a state 1912. From Spanish Arizonac, which is probably from a local name among the O'odham (Piman) people meaning "having a little spring." Alternative theory is that it derives from Basque arizonak "good oaks."
ark (n.) Look up ark at Dictionary.com
Old English earc, Old Northumbrian arc, mainly meaning Noah's but also the Ark of the Covenant, from Latin arca "large box, chest" (see arcane), the word used in the Vulgate. Also borrowed in Old High German (arahha, Modern German Arche). From the Noachian sense comes extended meaning "place of refuge" (17c.). As the name of a type of ship or boat, from late 15c. In 19c. U.S., especially a large, flat-bottomed river boat to move produce, livestock, etc. to market.
Arkansas Look up Arkansas at Dictionary.com
organized as a U.S. territory 1819, admitted as a state 1836; it was named for the Arkansas River, which was named for a Siouan tribe.
The spelling of the term represents a French plural, Arcansas, of a name applied to the Quapaw people who lived on the Arkansas River; their name was also written in early times as Akancea, Acansea, Acansa (Dickinson, 1995). This was not the name used by the Quapaws themselves, however. The term /akansa/ was applied to them by Algonquian speakers; this consists of /a-/, an Algonquian prefix found in the names of ethnic groups, plus /kká:ze, a Siouan term refering to members of the Dhegiha branch of the Siouan family. This stem is also the origin for the name of the Kansa tribe and of the state of Kansas; thus the placenames Arkansas and Kansas indirectly have the same origin. [William Bright, "Native American Placenames of the United States," 2004]
arm (v.) Look up arm at Dictionary.com
"to furnish with weapons," c. 1200, from Old French armer "provide weapons to; take up arms," or directly from Latin armare "furnish with arms," from arma "weapons," literally "tools, implements" of war (see arm (n.2)). Intransitive sense "provide oneself with weapons" in English is from c. 1400. Related: Armed; arming.
arm (n.2) Look up arm at Dictionary.com
"weapon," c. 1300, armes (plural) "weapons of a warrior," from Old French armes (plural), "arms, weapons; war, warfare" (11c.), from Latin arma "weapons" (including armor), literally "tools, implements (of war)," from PIE *armo-, suffixed form of root *ar- "to fit, join" (see arm (n.1)). The notion seems to be "that which is fitted together."

Meaning "branch of military service" is from 1798, hence "branch of any organization" (by 1952). Meaning "heraldic insignia" (in coat of arms, etc.) is early 14c., from Old French; originally they were borne on shields of fully armed knights or barons. To be up in arms figuratively is from 1704; to bear arms "do military service" is by 1640s.
arm (n.1) Look up arm at Dictionary.com
"upper limb of the human body," Old English earm, from Proto-Germanic *armaz (source also of Old Saxon, Danish, Swedish, Middle Dutch, German arm, Old Norse armr, Old Frisian erm), from PIE root *ar- "fit, join" (source also of Sanskrit irmah "arm," Armenian armukn "elbow," Old Prussian irmo "arm," Greek arthron "a joint," Latin armus "shoulder"). Arm of the sea was in Old English. Arm-twister "powerful persuader" is from 1915. Arm-wrestling is from 1899.
They wenten arme in arme yfere Into the gardyn [Chaucer]
arm-band (n.) Look up arm-band at Dictionary.com
1797, from arm (n.1) + band (n.1).
arm-rest (n.) Look up arm-rest at Dictionary.com
also armrest, 1848, from arm (n.1) + rest (n.).
armada (n.) Look up armada at Dictionary.com
"fleet of warships," 1530s (armado), from Spanish armada "an armed force," from Medieval Latin armata (see army). Current form of the word is from 1590s. The fleet sent by Philip II of Spain against England in 1588 was being called the Spanish Armada by 1613, the Invincible Armada by 1632.
armadillo (n.) Look up armadillo at Dictionary.com
burrowing mammal of the American tropics, 1570s, from Spanish armadillo, diminutive of armado "armored," from Latin armatus, past participle of armare "to arm, furnish with weapons" (see arm (n.2)). So called for its hard, plated shell.
Armageddon (n.) Look up Armageddon at Dictionary.com
"cataclysmic final conflict," 1811, figurative use of the place-name in Revelations xvi.16, site of the great and final conflict, from Hebrew Har Megiddon "Mount of Megiddo," a city in central Palestine, site of important Israelite battles.
armament (n.) Look up armament at Dictionary.com
1650s, "naval force equipped for war," from Latin armamentum "implement," from Latin armare "to arm, furnish with weapons" from arma "weapons" (including defensive armor), literally "tools, implements (of war);" see arm (n.2). Meaning "process of equipping for war" is from 1813.
armamentarium (n.) Look up armamentarium at Dictionary.com
"an armory," 1874, Latin, literally "little arsenal," from armamenta "implements, weapons" (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 "weapons," literally "tools, implements (of war);" 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, "chair with rests for the elbows," 1630s, from arm (n.1) + chair (n.). Another old name for it was elbow-chair (1650s). Adjectival sense, in reference to "criticism of matters in which the critic takes no active part," is from 1886.
armed (adj.) Look up armed at Dictionary.com
"equipped for battle," early 13c., past participle adjective from arm (v.).
Armenia Look up Armenia at Dictionary.com
late 14c., a place-name traced to 521 C.E., of uncertain origin. Armenian is from 1590s as "a native of Armenia;" as the name of the Indo-European language spoken there, by 1718; as an adjective, by 1727.
armful (n.) Look up armful at Dictionary.com
1570s, from arm (n.1) + -ful.
armilla (n.) Look up armilla at Dictionary.com
1706, "bracelet," from Latin armilla "bracelet, armlet, arm ring," from armus "shoulder, upper arm" (see arm (n.1)). Related: Armillary.
Arminian (adj.) Look up Arminian at Dictionary.com
1610s in reference to a Protestant sect, 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. As a noun from 1610s. Related: Arminianism.
armistice (n.) Look up armistice at Dictionary.com
"temporary suspension of hostilities by agreement of the parties," 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 *stā- "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) commemorated the end of the Great War of 1914-18 on Nov. 11, 1918 and memorialized the dead in that war. In Britain, after World War II, it merged with Remembrance Day. In U.S. (which suffered fewer casualties and had already a Memorial Day for the dead), Armistice Day became a national holiday in 1926; in 1954, to also honor living World War II and Korean War veterans, 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 more typically is expressed by unarmed or disarmed.
armlet (n.) Look up armlet at Dictionary.com
1530s, "metal band or ring worn around the upper arm," diminutive of arm (n.1) with -let. Compare bracelet. The Latin word was armilla. As "a small intrusion of the sea into the land," also 1530s.
armoire (n.) Look up armoire at Dictionary.com
"large wardrobe with doors and shelves," 1570s, from French armoire, from Old French armarie "cupboard, bookcase, reliquary" (12c., Modern French armoire), from Latin armarium "closet, chest, place for implements or tools," from arma "gear, tools, ship's tackle, weapons of war" (see arm (n.2)). The French word was borrowed earlier as ambry (late 14c.).
armor (v.) Look up armor at Dictionary.com
mid-15c., from armor (n.). Related: Armored; armoring.
armor (n.) Look up armor at Dictionary.com
c. 1300, "mail, defensive covering worn in combat," also, generally, "means of protection," from Old French armeure "weapons, armor" (12c.), from Latin armatura "arms, equipment," from arma "weapons" (including defensive armor), literally "tools, implements (of war)," see arm (n.2). Figurative use from mid-14c.

Meaning "military equipment generally," especially siege engines, is from late 14c. The word might have died with jousting if not for 19c. transference to metal-sheathed combat machinery beginning with U.S. Civil War ironclads (it is first attested in this sense in an 1855 report from the U.S. Congressional Committee on Naval Affairs). Meaning "protective envelope of an animal" is from c. 1600.
armor-plate (n.) Look up armor-plate at Dictionary.com
1860, from armor + plate (n.).
armorer (n.) Look up armorer at Dictionary.com
"maker or caretaker of armor," 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, "belonging to heraldry," from armory in the heraldic sense + -al (1).
Armorica Look up Armorica at Dictionary.com
ancient name for Brittany, from Gallo-Roman Aremorica, 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
c. 1300, "arms and weapons collectively; defensive armor;" see arm (n.2) + -ory. Meaning "place where arms are manufactured" is from mid-15c. (see armor + -y (1)). Also used in a sense of "arsenal" (mid-15c.); the sense of "science of heraldry" (late 15c.) is 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. Related: Armoured; armourer.
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., "hollow place under the shoulder," from arm (n.1) + pit (n.1). Arm-hole (early 14c.) was used in this sense but was obsolete by 18c. Another Middle English word was asselle (early 15c.), from Old French asselle, from Latin axilla. 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
"space equal to the length of a human arm," 1650s, from arm (n.1) + length. Figurative at arm's end is recorded from 1570s.
army (n.) Look up army at Dictionary.com
late 14c., "armed expedition," from Old French armée "armed troop, armed expedition" (14c.), from Medieval Latin armata "armed force," from Latin armata, fem. of armatus "armed, equipped, in arms," as a noun, "armed men, soldiers," 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; restriction to "land force" is by late 18c. Transferred meaning "host, multitude" is c. 1500. Meaning "body of men trained and equipped for war" is from 1550s. The Old English words were here (still preserved in derivatives such as 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. Army-ant is from 1863.
arnica (n.) Look up arnica at Dictionary.com
plant genus of the borage family, native to central Europe, 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," from Proto-Germanic *aron- "eagle" (from PIE root *or- "large bird;" see erne) + wald "power" (see wield).
Arnout (n.) Look up Arnout at Dictionary.com
"native of Albania," especially as part of the Turkish military forces, 1717, from Turkish Arnaut, from Modern Greek Arnabites, metathesized from Arbanites, rhotacized from *Albanites, from Medieval Latin Albanus (see Albania).
aroint (v.) Look up aroint at Dictionary.com
intransitive verb, c. 1600, used by Shakespeare (only in imperative, aroint thee! "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, spice" (a sense now obsolete), from Latin aroma "sweet odor," from Greek aroma "seasoning, a spice or sweet herb," which is of unknown origin. Meaning "fragrance, odor," especially an agreeable one, is from 1814. A hypercorrect plural is aromata. Related: Aromal.
aromatherapy (n.) Look up aromatherapy at Dictionary.com
by 1992, from French aromathérapie, which is attested from 1930s; see aroma + therapy.
aromatic (adj.) Look up aromatic at Dictionary.com
c. 1400, aromatyk, "giving out an aroma, fragrant, sweet," from Latin aromaticus, from Greek aromatikos, from aroma (genitive aromatos) "seasoning, sweet spice," which is of unknown origin.
aromatize (v.) Look up aromatize at Dictionary.com
"to perfume, render aromatic" (of medicines, the breath), early 15c., from Latin aromatizare, from Greek aromatizein "to spice," from aromat-, stem of aroma "seasoning, sweet spice," which is of unknown origin.
arose (v.) Look up arose at Dictionary.com
past tense of arise (v.).