Etymology
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infidel (n.)

mid-15c., "adherent of a religion opposed to Christianity," from Old French infidèle, from Latin infidelis "unfaithful, not to be trusted," in Late Latin "unbelieving" (in Medieval Latin also as a noun, "unbeliever"), from in- "not, opposite of" (see in- (1)) + fidelis "faithful" (from PIE root *bheidh- "to trust, confide, persuade").

Originally "a non-Christian" (especially a Saracen); later "one who does not believe in religion, disbeliever in religion generally" (1520s). Also used to translate Arabic qafir (see Kaffir), which is from a root meaning "to disbelieve, to deny," strictly referring to all non-Muslims but virtually synonymous with "Christian;" hence, from a Muslim or Jewish point of view, "a Christian" (1530s). As an adjective from mid-15c., "of a religion opposed to Christianity;" 1520s as "rejecting the Christian religion while accepting no other."

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infidelity (n.)
c. 1400, "want of faith, unbelief in religion; false belief, paganism;" also (early 15c.) "unfaithfulness or disloyalty to a person" (originally to a sovereign, by 16c. to a lover or spouse), from French infidélité (12c.) or directly from Latin infidelitatem (nominative infidelitas) "unfaithfulness, faithlessness," noun of quality from infidelis "unfaithful, unbelieving" (see infidel).
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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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unfaithful (adj.)
mid-14c., "acting falsely," from un- (1) "not" + faithful. In Middle English it also had a sense of "infidel, unbelieving, irreligious" (late 14c.). Sense of "not faithful in marriage" is attested from 1828. Related: Unfaithfully; unfaithfulness.
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miscreant (n.)

late 14c., "a heathen, a Saracen, a pagan, an unbeliever, a non-Christian," from miscreant (adj.) or from Old French mescreant, which also had a noun sense of "infidel, pagan, heretic." Sense of "villain, vile wretch, scoundrel" is first recorded 1590 in Spenser.

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giaour (n.)
1560s, Turkish term of contempt for non-Muslims, from Turkish pronunciation of Persian gaur, variant of gabr "infidel, fire-worshipper," originally applied to the adherents of the Zoroastrian religion. Used by the Turks especially of Christians, "and so commonly that it does not necessarily imply an insult" [Century Dictionary].
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Kaffir (n.)
1790, "infidel," earlier and also caffre (1670s), from Arabic kafir "unbeliever, infidel, impious wretch," with a literal sense of "one who does not admit (the blessings of God)," from kafara "to cover up, conceal, deny, blot out."

Technically, "a non-Muslim," but in Ottoman times it came to be used there almost exclusively as the disparaging word for "Christian." It also was used by Muslims in East Africa of the pagan black Africans; English missionaries then picked it up as an equivalent of "heathen" to refer to Bantus in South Africa (1731), from which use in English it came generally to mean "South African black" regardless of ethnicity, and to be a term of abuse at least since 1934.
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miscreant (adj.)

c. 1300, "non-Christian, misbelieving, pagan, infidel;" early 15c., "heretical, unbelieving," from Old French mescreant "disbelieving" (Modern French mécréant), from mes- "wrongly" (see mis- (2)) + creant, present participle of creire "believe," from Latin credere "to believe" (see credo). Meaning "villainous, vile, detestable" is from 1590s. Related: Miscreance; miscreancy.

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adverse (adj.)
late 14c., "contrary, opposing," from Old French advers, earlier avers (13c., Modern French adverse) "antagonistic, unfriendly, contrary, foreign" (as in gent avers "infidel race"), from Latin adversus "turned against, turned toward, fronting, facing," figuratively "hostile, adverse, unfavorable," past participle of advertere "to turn toward," from ad "to" (see ad-) + vertere "to turn, turn back; be turned; convert, transform, translate; be changed" (from PIE root *wer- (2) "to turn, bend"). For distinction of use, see averse. Related: Adversely.
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