Fiance

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mid 19th century: from French, past participle of fiancer ‘betroth’, from Old French fiance ‘a promise’, based on Latin fidere ‘to trust’.

wiktionary

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From French fiancé. The verb is from fiancer.

etymonline

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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."