What is the X in X-mas?
"woman to whom one is betrothed," 1853, from French fianceé, fem. of fiancé, past participle of fiancer "to betroth," from fiance "a promise, trust," from fier "to trust," from Vulgar Latin *fidare (see affiance). Has all but expelled native betrothed. The verb fiance, now obsolete, was used c.1450-1600 for "to engage to be married."