“Uncle Chris felt a touch of embarrassment. It occurred to him that he had been betrayed by his mercurial temperament into an attitude which, considering the circumstances, was perhaps a trifle too jubilant. He gave his moustache a pull, and reverted to the minor key.” — From P.G. Wodehouse's 1921 novel, Jill the Reckless
“The market, trendless and mercurial, had to digest conflicting news about the economy.” — From an Associated Press article by Christina Rexrode, August 9, 2012
The Roman god Mercury (“Mercurius” in Latin) was the messenger and herald of the gods and also the god of merchants and thieves. (His counterpart in Greek mythology is Hermes.) He was noted for his eloquence, swiftness, and cunning, and the Romans named what appeared to them to be the fastest-moving planet in his honor. The Latin adjective derived from his name, “mercurialis,” meaning “of or relating to Mercury,” was borrowed into English in the 14th century as “mercurial.” Although the adjective initially meant “born under the planet Mercury,” it came to mean also “having qualities of eloquence, ingenuity, or thievishness attributed to the god Mercury or the influence of the planet Mercury,” and then “unpredictably changeable.”
From Merriam-Webster Online at www.Merriam-Webster.com.