fiance 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."