Etymology
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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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disloyalty (n.)

"want of loyalty, unfaithful behavior," early 15c., disloialte, from a variant of Old French desloiaute, desleauté "disloyalty, faithlessness, marital infidelity," from desloial, desleal "treacherous, false, deceitful" (Modern French déloyal), from des- "not, opposite of" (see dis-) + loial "of good quality; faithful; honorable; law-abiding; legitimate, born in wedlock," from Latin legalem, from lex "law" (see legal). Since c. 1600 especially "violation of allegiance or duty to a state or sovereign."

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