Etymology
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heel (n.1)

"back of the foot," Old English hela, from Proto-Germanic *hanhilon (source also of Old Norse hæll, Old Frisian hel, Dutch hiel), from PIE *kenk- (3) "heel, bend of the knee" (source also of Old English hoh "hock").

Meaning "back of a shoe or boot" is c. 1400. Down at heels (1732) refers to heels of boots or shoes worn down and the owner too poor to replace them. For Achilles' heel "only vulnerable spot" see Achilles. To fight with (one's) heels (fighten with heles) in Middle English meant "to run away."

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filigree (n.)
1690s, shortening of filigreen (1660s), from French filigrane "filigree" (17c.), from Italian filigrana, from Latin filum "thread, wire" (from PIE root *gwhi- "thread, tendon") + granum "grain" (from PIE root *gre-no- "grain"). Related: Filigreed.
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nervose (adj.)

"having nerves," in any sense, 1753, from Latin nervosus "full of sinews or fibers," from nervus "sinew, tendon" (see nerve (n.)).

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nerve (n.)

late 14c., nerve, nerf, "sinew, tendon, hard cord of the body" (a sense now obsolete), also "fiber or bundle of fibers that convey the capacity to feel or move from the brain or spinal cord to the body," from Old French nerf and directly from Medieval Latin nervus "a nerve," from Latin nervus "sinew, tendon; cord, bowstring, string of a musical instrument," metathesis of pre-Latin *neuros, from PIE *(s)neu- "tendon, sinew" (source also of Sanskrit snavan- "band, sinew," Armenian neard "sinew," Greek neuron "sinew, tendon," in Galen "nerve").

The late medieval surgeons understood the nature and function of the nerves and often used nervus to denote a `nerve' in the modern sense, as well as to denote a `tendon'. There appears to have been some confusion, however, between nerves and tendons; hence, a number of instances in which nervus may be interpreted in either way or in both ways simultaneously. [Middle English Compendium] 

The secondary senses developed from meaning "strength, vigor; force, energy" (c. 1600), from the "sinew" sense. Hence the non-scientific sense with reference to feeling or courage, first attested c. 1600 (as in nerves of steel, 1869) and that of "coolness in the face of danger, fortitude under trying or critical circumstances" is by 1809. The bad sense "impudence, boldness, cheek" (originally slang) is by 1887. Latin nervus also had a figurative sense of "vigor, force, power, strength," as did Greek neuron. From the neurological sense come Nerves "condition of hysterical nervousness," attested by 1890, perhaps from 1792. To get on (someone's) nerves is from 1895. War of nerves "psychological warfare" is from 1915.

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filament (n.)
"fine untwisted thread, separate fibril," 1590s, from Modern Latin filamentum, from Late Latin filare "to spin, draw out in a long line," from Latin filum "thread" (from PIE root *gwhi- "thread, tendon"). As the name of the incandescent element in a light-bulb, from 1881.
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hamstring (v.)

1640s, "to disable, render useless," a figurative verbal extension from hamstring (n.) "tendon at the back of the knee." Cutting this would render a person or animal lame. Literal sense of the verb is attested from 1670s. Since it is a verb from a noun-noun compound, hamstrung as a past participle is technically incorrect.

[I]n hamstring, -string is not the verb string; we do not string the ham, but do something to the tendon called the hamstring; the verb, that is, is made not from the two words ham & string, but from the noun hamstring. It must therefore make hamstringed. [Fowler]
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neuro- 

before vowels neur-, word-forming element meaning "pertaining to a nerve or nerves or the nervous system," from Greek neura "nerve" (Galen), originally "sinew, bowstring," also neuron "sinew, string (of a bow or musical instrument); cord; penis;" in plural "strength, vigor," from PIE *(s)neuro- "tendon, sinew" (see nerve (n.)). In Greek, puppets were neurospastos, literally "drawn by strings."

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profile (n.)

1650s, "a drawing of the outline of anything," especially "a representation of the human face in side view," from older Italian profilo "a drawing in outline," from profilare "to draw in outline," from pro "forth" (from PIE root *per- (1) "forward") + filare "draw out, spin," from Late Latin filare "to spin, draw out a line," from filum "thread" (from PIE root *gwhi- "thread, tendon"). Meaning "a side view" is from 1660s. Meaning "biographical sketch, character study" is from 1734.

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*gwhi- 
*gwhī-, Proto-Indo-European root meaning "thread, tendon."

It forms all or part of: defile (n.) "narrow passage;" enfilade; filament; file (v.1) "place (papers) in consecutive order for future reference;" filigree; filipendulous; fillet; profile.

It is the hypothetical source of/evidence for its existence is provided by: Avestan jya- "bowstring;" Latin filum "a thread, string;" Armenian jil "sinew, string, line;" Lithuanian gysla "vein, sinew;" Old Church Slavonic zila "vein."
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defile (n.)

"narrow passage in a mountain region," 1640s, especially in a military sense, "a narrow passage down which troops can march only in single file," from French défilé, noun use of past participle of défiler "march by files" (17c.), from de- "off" (see de-) + file "row," from Latin filum "thread" (from PIE root *gwhi- "thread, tendon"). The verb, "to march off in a line or file," is by 1705, from French défiler.

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