Etymology
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arc (n.)

late 14c., "part of a curved line," originally in reference to the sun's apparent motion across the sky, from Old French arc "bow, arch, vault" (12c.), from Latin arcus "a bow, arch," from Proto-Italic *arkwo- "bow."

This has Germanic cognates in Gothic arhvazna, Old English earh, Old Norse ör "arrow," from Proto-Germanic *arkw-o- "belonging to a bow." It also has cognates in Greek arkeuthos, Latvian ercis "juniper," Russian rakita, Czech rokyta, Serbo-Croatian rakita "brittle willow." De Vaan sees an Italo-Germanic word for "bow" which can be connected with Balto-Slavic and Greek words for "willow" and "juniper" "under the well-founded assumption that the flexible twigs of juniper or willow were used as bows." The Balto-Slavic and Greek forms point to *arku-; "as with many plant names, this is likely to be a non-IE loanword."

The electrical sense is attested from 1821.

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arc (v.)

1882, in the electrical sense, from arc (n.). Meaning "to move in an arc" is attested by 1940. Related: Arced; arcing.

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arc-light (n.)

"light produced by an electric arc," 1871, from arc (n.) + light (n.). Related: Arc-lamp.

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arcuate (adj.)

"bent like a bow," 1620s, from Latin arcuatus "bow-like, arched," past participle of arcuare "to bend like a bow," from arcus "a bow" (see arc (n.)). Related: Arcuration.

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arcade (n.)

1731, "vaulted space" (as arcado from 1640s), via French arcade, which probably is from Italian arcata "arch of a bridge," from arco "arc," from Latin arcus "a bow, arch" (see arc (n.)).

The English word was applied to passages formed by a succession of arches supported on piers or pillars, avenues of trees, and ultimately to any covered avenue (1731), especially one lined with shops (1795) or amusements; hence arcade game (1977).

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archer (n.)

"one who shoots arrows from a (long) bow," late 13c., from Anglo-French archer, Old French archier "archer; bow-maker," from Late Latin arcarius, alteration of Latin arcuarius "maker of bows," from arcus "bow" (see arc (n.)). The classical Latin word was arquites "archers;" the Greeks shunned archery as an unmanly tactic, and the Romans seem to have had little appreciation for it until their later encounters with mounted barbarian archers.

Also a 17c. name for the bishop in chess. As a type of tropical fish, 1834, from its shooting drops of water at insects. For "archer" Middle English had bowman, also scutte, from Old English scytta, also bender (which also meant "maker of bows," a surname).

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arbalest (n.)

type of crossbow, also arbalist, c. 1300, from Old French arbaleste "large crossbow with a crank" (12c., Modern French arbalète), from Vulgar Latin *arbalista, from Late Latin arcuballista "catapult," from Latin arcus "bow" (see arc (n.)) + ballista "machine for throwing projectiles" (from PIE root *gwele- "to throw, reach"). German Armbrust is from the same French word but mangled by folk etymology. Related: Arbalester.

The missile of the arbalist was discharged with such force as to penetrate ordinary armor, and the weapon was considered so deadly as to be prohibited by a council of the church except in warfare against infidels. [Century Dictionary]
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arch (n.)

"structure (in a building, bridge, etc.) in the shape of a curve that stands when supported only a the extremities," c. 1300, from Old French arche "arch of a bridge, arcade" (12c.), from Latin arcus "a bow" (see arc (n.)). It largely replaced native bow (n.1) in this sense.

Originally architectural in English; transferred by early 15c. to anything having a curved form (eyebrows, feet, etc.). The commemorative or monumental arch is attested in English from late 14c.

Compare Middle English Seinte Marie Chirche of the Arches (c. 1300) in London, later known as St. Mary-le-Bow, site of an ecclesiastical court, so called for the arches that supported its steeple (the modern church is by Sir Christopher Wren, rebuilt after the Great Fire of 1666).

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arrow (n.)

"slender, pointed missile weapon, made to be shot from a bow," early 14c., from Old English arwan, earlier earh "arrow," possibly borrowed from Old Norse ör (genitive örvar), from Proto-Germanic *arkhwo (source also of Gothic arhwanza), from PIE root *arku-, source of Latin arcus (see arc (n.)). The etymological sense would be "the thing belonging to the bow." The meaning "a mark like an arrow" in cartography, etc. is from 1834.

It was a rare word in Old English. More common words for "arrow" were stræl (which is cognate with the word still common in Slavic and once prevalent in Germanic, related to words meaning "flash, streak") and fla, flan (the -n perhaps mistaken for a plural inflection), from Old Norse, a North Germanic word, perhaps originally with the sense of "splinter." Stræl disappeared by 1200; fla became flo in early Middle English and lingered in Scottish until after 1500.

Robyn bent his joly bowe,
Therein he set a flo.
["Robyn and Gandelyn," in a minstrel book from c. 1450 in the British Museum]
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radian (n.)

"angle subtended at the center of a circle by an arc equal in length to the radius," 1879, from radius.

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