What is the origin of mercurial?
The English adjective mercurial ultimately comes from the Latin adjective mercuriālis “of or pertaining to Mercurius“ (i.e., the god Mercury), whose original function was as god of commerce, transporters of goods (especially of grain), and shopkeepers. Latin also has the plural noun, derived from the adjective, Mercuriālēs, the name of a guild of merchants. Mercurius is related to merx (stem merc-) “goods, wares, commodities” (and the ultimate source of English merchant and merchandise). By classical times Mercury was completely identified with the Greek god Hermes—the messenger of the gods because he was fast-moving, and always on the move, negotiating, fast-talking, making deals, flimflamming, playing tricks. Mercurius also acquired the meaning “pertaining to the planet Mercury” (Stella Mercuriī, “Star of Mercury,” a translation of Greek astḕr toû Hermoû), the fastest moving of the planets. Mercurial entered English in the 14th century in the sense “pertaining to the planet Mercury.”