Etymology
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affidavit (n.)
"written declaration upon an oath," 1590s, from Medieval Latin affidavit, literally "he has stated on oath," third person singular perfective of affidare "to trust; to make an oath," from Latin ad "to" (see ad-) + fidare "to trust," from fidus "faithful," from PIE root *bheidh- "to trust, confide, persuade." So called from being the first word of sworn statements.
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federate (v.)
1814 (implied in federated), a back-formation from federation, or else from Latin foederatus "leagued, federated, combined; having a treaty, bound by treaty," past participle of foederare "to establish by treaty," from foedus "covenant, treaty, alliance" (from PIE root *bheidh- "to trust, confide, persuade"). Related: Federating. As an adjective, by 1710.
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defiance (n.)
Origin and meaning of defiance

c. 1300, defiaunce, "a challenge to fight, invitation to combat," from Old French desfiance "challenge, declaration of war," from desfiant, present participle of desfier "to challenge, defy, provoke; renounce (a belief), repudiate (a vow, etc.)," from Vulgar Latin *disfidare "renounce one's faith," from Latin dis- "away" (see dis-) + fidus "faithful" (from PIE root *bheidh- "to trust, confide, persuade"). By 1710 as "contempt of opposition or danger."

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solifidian (n.)
"one who believes in salvation by faith alone" (based on Luther's translation of Romans iii.28), 1590s, Reformation coinage from Latin solus "alone" (see sole (adj.)) + fides "faith" (from PIE root *bheidh- "to trust, confide, persuade"). As an adjective from c. 1600. Related: Solifidianism
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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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fiancee (n.)
"woman to whom one is betrothed," 1844 (1837 as a French word in English), from French fiancée, fem. of fiancé, past participle of fiancer "to betroth," from fiance "a promise, trust," from fier "to trust," from Vulgar Latin *fidare "to trust," from Latin fidus "faithful" from the same root as fides "faith" (from PIE root *bheidh- "to trust, confide, persuade"). It has all but expelled native betrothed. The English verb fiance, now obsolete, was used c. 1450-1600 for "to engage to be married."
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diffident (adj.)
Origin and meaning of diffident

mid-15c., "distrustful, wanting confidence in another's power," from Latin diffidentem (nominative diffidens), present participle of diffidere "to mistrust, lack confidence," from dis- "away" (see dis-) + fidere "to trust" (from PIE root *bheidh- "to trust, confide, persuade"). Original sense is obsolete; modern sense of "distrustful of oneself, not confident" is by 1713. Compare diffidence. Related: Diffidently.

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confederate (v.)
Origin and meaning of confederate

1530s, "to unite in a league or alliance," from Late Latin confoederatus, past participle of confoederare "to unite by a league," from assimilated form of com "with, together" (see con-) + foederare, from foedus (genitive foederis) "a league," from suffixed form of PIE root *bheidh- "to trust, confide, persuade."

The older verb was confeder (late 14c.), from Old French confederer, Medieval Latin confederare. Related: Confederated; confederating.

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affiance (v.)
1520s, "to promise," from Old French afiancier "to pledge, promise, give one's word," from afiance (n.) "confidence, trust," from afier "to trust," from Late Latin affidare, from ad "to" (see ad-) + fidare "to trust," from fidus "faithful," from PIE root *bheidh- "to trust, confide, persuade."

From mid-16c. especially "to promise in marriage." The earlier form of the word was affy (Middle English affien "to trust, have faith; have faith in" c. 1300), from Old French afier. Related: Affianced; affiancing.
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auto-da-fe (n.)
"sentence passed by the Inquisition" (plural autos-da-fé), 1723, from Portuguese auto-da-fé "judicial sentence, act of the faith," especially the public burning of a heretic, from Latin actus de fide. The elements are auto "a play," in law, "an order, decree, sentence," from Latin actus (see act (v.)), de "from, of" (see de), fides "faith" (from PIE root *bheidh- "to trust, confide, persuade"). The Spanish form is auto-de-fe, but the Portuguese form took hold in English, perhaps through popular accounts of the executions following the earthquake of 1755.
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