Etymology
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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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fiance (n.)
"man to whom one is betrothed," 1864 (by 1826 as a French word in English), from French fiancé, past participle of fiancer "to betroth" (see fiancee). Borrowed earlier in Middle English as "confidence, trust; word of honor."
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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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-ee 
word-forming element in legal English (and in imitation of it), representing the Anglo-French ending of past participles used as nouns (compare -y (3)). As these sometimes were coupled with agent nouns in -or, the two suffixes came to be used as a pair to denote the initiator and the recipient of an action.

Not to be confused with the French -ée that is a feminine noun ending (as in fiancée), which is from Latin -ata.
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