What is the origin of au fond?

The French adverb phrase au fond, “thoroughly; in reality; fundamentally,” literally “at the bottom, to the bottom,” has been in English for more than 200 years; yet its French pronunciation in English shows that it is still unnaturalized. The French phrase is composed of au “at the, to the,” from Old French al, which is a contraction of a le, from Latin ad “to” and illum “that” (illum and its relatives become the definite article in most Romance languages). The French noun fond “bottom, floor, background (for lacework)” comes from Latin fundus “bottom, base, depths, farm, country estate.” The Latin noun is the source of the verb fundāre “to lay a foundation,” which becomes fonder in Old French, founden, fonden, funden in Middle English, and found, i.e., “establish firmly,” in modern English. Au fond entered English toward the end of the 18th century.