Quick to change moods or change one’s mind, having an unpredictable temperament.
→ Synonyms of mercurial include flighty, impulsive, fickle, capricious (which properly rhymes with delicious; it’s word 11 of Level 1), volatile (word 47 of Level 4), erratic, and protean (PROH-tee-in).
Antonyms include stable, fixed, steadfast, invariable, and immutable.
Does anything about the word mercurial sound familiar? Can you guess its derivation? If you’re thinking that mercurial is related to the word mercury, then you are a sagacious person, both in the current sense of wise, shrewd, perceptive, and in the obsolete sense of quick in picking up a scent—in this case, an etymological scent.
The ancient Roman god Mercury, known to the Greeks as Hermes (HUR-meez), was the messenger or courier of the gods, but he had many other responsibilities as well. He was the deity (DEE-i-tee) who conducted the souls of the dead to the underworld, and also the god of commerce, travel, eloquence, and thievery. (Those ancient Greeks and Romans covered all the bases, didn’t they?)
Mercury is usually depicted wearing a winged helmet and winged sandals to show his fleet-footedness, and as Hermes he also carried a winged staff with two serpents coiled around it. That staff, which now serves as the symbol of the medical profession, is called a caduceus (kuh-D(Y)OO-see-us).
I’m sure that doctors today view the caduceus as a symbol of their devotion to providing swift, efficient health care, but I must confess I find it nothing short of hilarious that the medical profession has chosen a symbol from an ancient god who governed commerce, travel, eloquence, and thievery, and who escorted the dead to their final resting place.
Because of the various hats worn by the god Mercury, the adjective mercurial has been used to mean everything from swift, quick-witted, and eloquent to shrewd, clever, and thieving. Dictionaries still list these words under the definition “having the characteristics attributed to the god Mercury,” but in current usage the word most often is used to mean like the element mercury, which is also called quicksilver. As you know, mercury is used in thermometers, and it is highly reponsive to changes in temperature. Like the mercury in a thermometer, that which is mercurial is changeable, fickle, or capricious. The mercurial person has an unpredictable temperament and is quick to change moods.