Etymology
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fiber (n.)
late 14c., fibre "a lobe of the liver," also "entrails," from Medieval Latin fibre, from Latin fibra "a fiber, filament; entrails," which is of uncertain origin, perhaps related to Latin filum "a thread, string" (from PIE root *gwhi- "thread, tendon") or to Latin findere "to split" (from PIE root *bheid- "to split").

Meaning "thread-like structure in animal bodies" is from c. 1600 (in plants, 1660s); hence figurative use in reference to force or toughness (1630s). As "textile material," 1827. Fiberboard is from 1897; Fiberglas is attested from 1937, U.S. registered trademark name; in generic use, with lower-case f- and double -s, by 1941. Fiber optics is from 1956.
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cord (n.)

c. 1300, corde, "a string or small rope composed of several strands twisted or woven together; bowstring, hangman's rope," from Old French corde "rope, string, twist, cord," from Latin chorda "string of a musical instrument, cat-gut," from Greek khorde "string, catgut, chord, cord," from PIE root *ghere- "intestine."

Also from c. 1300 as "string of a musical instrument." From c. 1400 as "a tendon or muscle." Figurative sense of "anything which binds or restrains" is from late 14c. Meaning "raised, cord-like rib on the surface of cloth" is from 1776. As a measure of wood of 128 cubic feet (eight feet long, four feet high and wide) first recorded 1610s, so called because it was measured with a cord of rope.

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file (v.1)

"place (papers) in consecutive order for future reference," mid-15c., from Old French filer "string documents on a thread or wire for preservation or reference" (15c.), earlier "to spin thread," from fil "thread, string" (12c.), from Latin filum "a thread, string; thread of fate; cord, filament," from PIE *gwhis-lom, suffixed form of root *gwhi- "thread, tendon." The notion is of documents hung up on a line in consecutive order for ease of reference.

File (filacium) is a threed or wyer, whereon writs, or other exhibits in courts, are fastened for the better keeping of them. [Cowel, "The Interpreter," 1607]

Methods have become more sophisticated, but the word has stuck. Meaning "place among the records of a court or office" is from 1510s; of newspaper reporters sending in stories, 1954. Intransitive sense "march in a line (as soldiers do) one after another" is from 1610s. Related: Filed; filing.

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*ten- 
Proto-Indo-European root meaning "to stretch," with derivatives meaning "something stretched, a string; thin."

It forms all or part of: abstain; abstention; abstinence; abstinent; atelectasis; attend; attenuate; attenuation; baritone; catatonia; catatonic; contain; contend; continue; detain; detente; detention; diatonic; distend; entertain; extend; extenuate; hypotenuse; hypotonia; intend; intone (v.1) "to sing, chant;" isotonic; lieutenant; locum-tenens; maintain; monotony; neoteny; obtain; ostensible; peritoneum; pertain; pertinacious; portend; pretend; rein; retain; retinue; sitar; subtend; sustain; tantra; telangiectasia; temple (n.1) "building for worship;" temple (n.2) "flattened area on either side of the forehead;" temporal; tenable; tenacious; tenacity; tenant; tend (v.1) "to incline, to move in a certain direction;" tendency; tender (adj.) "soft, easily injured;" tender (v.) "to offer formally;" tendon; tendril; tenement; tenesmus; tenet; tennis; tenon; tenor; tense (adj.) "stretched tight;" tensile; tension; tensor; tent (n.) "portable shelter;" tenterhooks; tenuous; tenure; tetanus; thin; tone; tonic.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit tantram "loom," tanoti "stretches, lasts," tanuh "thin," literally "stretched out;" Persian tar "string;" Lithuanian tankus "compact," i.e. "tightened;" Greek teinein "to stretch," tasis "a stretching, tension," tenos "sinew," tetanos "stiff, rigid," tonos "string," hence "sound, pitch;" Latin tenere "to hold, grasp, keep, have possession, maintain," tendere "to stretch," tenuis "thin, rare, fine;" Old Church Slavonic tento "cord;" Old English þynne "thin."
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