The Proto Indo Europeans of the Asian steppe had a word, bheidh, meaning "advise, encourage, command", much the same as its rather stuffy-sounding English descendant bid (as in "He bade me come inside," "I will do as you bid"). The word came into Latin as fidare, "to trust", from which was derived the noun fides, "trust, confidence, faith." This went into Old French as feid, and became faith in Middle English. Meanwhile Latin also had a derived form, adfidare, meaning roughly "to state on faith," which descended into Medeival Latin as affadavit "he has sworn". The two paired terms went into Old French as afier and fier; they were converted into the French nouns afiance and fiance, "confidence, trust", and two verbs in turn derived from that, afiancier and fiancer, "to promise". They were borrowed into English as the verb affiance ("to become betrothed") and the noun fiancé (the one promising). The feminine form of fiancé was borrowed as fiancée.
The two words replaced the old Anglo Saxon betrothed, which literally means "become pledged", and is related to truth, in the sense that a promise literally makes something true.
The words fiancé, fiancée have a spiritual trajectory of high, tense freedom being gathered up and grounded, with a release of sunlight, energy, and motion.