Etymology
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fidelity (n.)

early 15c., "faithfulness, devotion," from Old French fidélité (15c.), from Latin fidelitatem (nominative fidelitas) "faithfulness, adherence, trustiness," from fidelis "faithful, true, trusty, sincere," from fides "faith" (from PIE root *bheidh- "to trust, confide, persuade"). From 1530s as "faithful adherence to truth or reality;" specifically of sound reproduction from 1878.

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hi-fi (adj.)

1947, abbreviation of high-fidelity (1934), a descriptive term of radio receivers in reference to their quality of sound reproduction. Hi as an advertiser's phonological shortening of high (adj.) is attested by 1914. Fidelity in the sense "faithful reproduction of sound" is from 1878.

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feal (adj.)
"faithful," 1560s, not found in Middle English but apparently from Old French feal "faithful, loyal, true, sincere," collateral form of feeil, from Latin fidelis "loyal" (see fidelity).
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*bheidh- 
Proto-Indo-European root meaning "to trust, confide, persuade."

It forms all or part of: abide; abode; affiance; affidavit; auto-da-fe; bide; bona fide; confederate; confidant; confide; confidence; confident; defiance; defy; diffidence; diffident; faith; fealty; federal; federate; federation; fiancee; fideism; fidelity; fiducial; fiduciary; infidel; infidelity; nullifidian; perfidy; solifidian.

It is the hypothetical source of/evidence for its existence is provided by: Greek pistis "faith, confidence, honesty;" Latin fides "trust, faith, confidence, reliance, credence, belief;" Albanian be "oath," bindem "to be convinced, believe;" Old Church Slavonic beda "distress, necessity," bediti "to force, persuade;" Old English biddan "to ask, beg, pray," German bitten "to ask."
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fealty (n.)
c. 1300, feaute, from Old French feauté, earlier fealte, "loyalty, fidelity; homage sworn by a vassal to his overlord; faithfulness," from Latin fidelitatem (nominative fidelitas) "faithfulness, fidelity," from fidelis "loyal, faithful" (from PIE root *bheidh- "to trust, confide, persuade").
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audiophile (n.)

"enthusiast of high-quality sound reproduction," 1951, originally in "High Fidelity" magazine, from audio- + -phile.

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constancy (n.)

1520s, "fixedness or firmness of mind," a fuller form of constance (q.v.) with abstract noun suffix -cy. Meaning "faithfulness, fidelity" (to a person or cause) is from 1540s; that of quality of immutability, a permanent state" is from c. 1600.

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soothfast (adj.)

"truthful," Old English soðfæst "true, trustworthy, honest, just, righteous;" see sooth (n.) + fast (adj). Related: Old English soðfæstnes "truthfulness, fairness, fidelity;" soðfæstlic "true, sincere;" soðfæstlice "truly, honestly."

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-ality 

word-forming element; see -al (1) + -ity. Originally also in reduced form -alty, especially in words from French (mayoralty, etc.), hence the occasional doublet such as fealty/fidelity, realty/reality, specialty/speciality, loyalty/legality.

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naturalism (n.)

1630s, "action based on natural instincts," from natural (adj.) + -ism. In philosophy, as a view of the world and humanity's relationship to it involving natural forces only (and excluding spiritualism and superstition), from 1750. As a tendency in art and literature, "conformity to nature or reality, but without slavish fidelity to it," from 1850.

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