Etymology
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Words related to *ar-

adorn (v.)
late 14c., aournen, later adornen, "to decorate, embellish," also "be an ornament to," from Old French aorner "to order, arrange, dispose, equip; adorn," from Latin adornare "equip, provide, furnish;" also "decorate, embellish," from ad "to" (see ad-) + ornare "prepare, furnish, adorn, fit out," from stem of ordo "row, rank, series, arrangement" (see order (n.)). The -d- was reinserted by French scribes 14c. and in English from late 15c. Related: Adorned; adorning.
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alarm (n.)
late 14c., "a call to arms in the face of danger or an enemy," from Old French alarme (14c.), from Italian all'arme "to arms!" (literally "to the arms"); a contraction of phrase alle arme. Alle is itself a contraction of a "to" (from Latin ad; see ad-) + le, from Latin illas, fem. accusative plural of ille "the" (see le); with arme, from Latin arma "weapons" (including armor), literally "tools, implements (of war)," from PIE root *ar- "to fit together."

The interjection came to be used as the word for the call or warning (compare alert). Extended 16c. to "any sound to warn of danger or to arouse," and to the device that gives it. From mid-15c. as "a state of fearful surprise;" weakened sense of "apprehension, unease" is from 1833. Variant alarum (mid-15c.) is due to the rolling -r- in the vocalized form. Sometimes in early years Englished as all-arm. Alarm clock is attested from 1690s (as A Larum clock).
aristarchy (n.)
"government by the best men; body of worthies constituting a government," 1889, from Greek aristarkhia, from aristos "best" (see aristo-) + -arkhia "government" (see -archy).
aristo- 
word-forming element meaning "best," also "of the aristocracy," from Greek aristos "best of its kind, noblest, bravest, most virtuous" (of persons, animals, things), originally "most fitting," from PIE *ar(ə)-isto-, suffixed (superlative) form of root *ar- "to fit together."
aristocracy (n.)

1560s, "government by those who are the best citizens," from French aristocracie (Modern French aristocratie), from Late Latin aristocratia, from Greek aristokratia "government or rule of the best; an aristocracy," from aristos "best of its kind, noblest, bravest, most virtuous" (see aristo-) + abstract noun from kratos "rule, power" (see -cracy).

In early use contrasted with monarchy; after the French and American revolutions, with democracy. Meaning "rule by a privileged class, oligarchy, government by those distinguished by rank and wealth" (best-born or best-favored by fortune) is from 1570s and became paramount 17c. Hence, the meaning "patrician order, the class of hereditary nobles" (1610s) and, generally, "persons notably superior in any way, taken collectively" (1650s).

arm (n.1)

"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- "to fit together" (source also of Sanskrit irmah "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 (n.2)

"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 *ar(ə)mo-, suffixed form of root *ar- "to fit together." 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.

armada (n.)
"fleet of warships," 1530s (armado), from Spanish armada "an armed force," from Medieval Latin armata "armed force" (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.)
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," from arma "weapons" (including defensive armor), literally "tools, implements (of war);" see arm (n.2). So called for its hard, plated shell.
armament (n.)
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.