Online Etymology Dictionary

aristocrat (n.): 1789, from French aristocrate, a word of the Revolution, a back-formation from aristocratie (see aristocracy).
aristocratic (adj.): c. 1600, "pertaining to aristocracy," from French aristocratique, from Greek aristokratikos "belonging to the rule of the best," from aristokratia (see aristocracy). Meaning "grand, stylish" is from 1845.
aristology (n.): "science of dining," 1835, from Greek ariston "breakfast" (see ere; so called from being the early meal of the day) + -ology. Related: Aristological.
Aristotelian (adj.): also Aristotelean, c. 1600, of or pertaining to the person or teachings of Greek philosopher Aristotle (384-322 B.C.E.).
arithmancy (n.): "divination by numbers," 1570s, from Greek arithmos "number" (see arithmetic) + -manteia "divination" (see -mancy).
arithmetic (n.): 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).

Originally in English also arsmetrik, on folk etymology from Medieval Latin ars metrica; spelling corrected early 16c. Replaced native tælcræft, literally "tell-craft."
arithmetical (adj.): 1540s; see arithmetic + -al (1). Related: Arithmetically (late 15c.). In modern use, opposed to geometrical.
arithmocracy (n.): "rule by numerical majority," 1850, from Greek arithmos "number, counting, amount" (see arithmetic) + -cracy. Related: Arithmocratic; arithmocratical.
arithmomania (n.): "compulsive desire to count objects and make calculations," 1890, from French arithmomanie, from Greek arithmos "number, counting, amount" (see arithmetic) + French -manie (see mania). Related: Arithmomaniac.
Arizona: 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. From Spanish Arizonac, 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.): Old English earc, mainly meaning Noah's but also the Ark of the Covenant, from Latin arca "large box, chest" (see arcane). 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: organized as a U.S. territory 1819 (admitted as a state 1836), 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 (n.1): "upper limb," Old English earm "arm," 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 1938. Arm-wrestling is from 1899.
They wenten arme in arme yfere Into the gardyn [Chaucer]
arm (n.2): "weapon," c. 1300, armes (plural) "weapons of a warrior," from Old French armes (plural), "arms, war, warfare," mid-13c., from Latin arma "weapons" (including armor), literally "tools, implements (of war)," from PIE root *ar- "fit, join" (see arm (n.1)). The notion seems to be "that which is fitted together." Meaning "heraldic insignia" (in coat of arms, etc.) is early 14c.; originally they were borne on shields of fully armed knights or barons.
arm (v.): "to furnish with weapons," c. 1200, from Old French armer or directly from Latin armare, from arma (see arm (n.2)). Related: Armed; arming.
armada (n.): "fleet of warships," 1530s (erroneously, as armado), from Spanish armada "an armed force," from Medieval Latin armata (see army). Especially of the "Invincible Armada" of Philip II of Spain (1588). Current form of the word is from 1590s. The Spanish Armada of 1588 was so called by 1613; Invincible Armada attested by 1632.
armadillo (n.): 1570s, from Spanish armadillo, diminutive of armado "armored," from Latin armatus, past participle of armare "to arm" (see arm (n.2)). So called for its hard, plated shell.
Armageddon (n.): "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.): 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.): 1874, Latin, literally "little arsenal," from armamentum (see armament). Englished as armamentary (1731).
armature (n.): 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.): 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.): "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 which is 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 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) 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) with -let.
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-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.): "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," which is 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," which is 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.).