Merriam-Webster Logo
  • Dictionary
  • Thesaurus
  • Medical
  • Scrabble
  • Spanish Central
  • Learner's Dictionary


adjective mer·cu·ri·al \(ˌ)mər-ˈkyu̇r-ē-əl\

Simple Definition of mercurial

  • : changing moods quickly and often

  • : changing often : very changeable

  • : very lively and quick

Source: Merriam-Webster's Learner's Dictionary

Full Definition of mercurial

  1. 1 :  of, relating to, or born under the planet Mercury

  2. 2 :  having qualities of eloquence, ingenuity, or thievishness attributed to the god Mercury or to the influence of the planet Mercury

  3. 3 :  characterized by rapid and unpredictable changeableness of mood <a mercurial temper>

  4. 4 :  of, relating to, containing, or caused by mercury

mercurially play \-ē-ə-lē\ adverb
mercurialness noun

Examples of mercurial in a sentence

  1. Few moments in English history have been more hungry for the future, its mercurial possibilities and its hope of richness, than the spring of 1603. —Adam Nicolson, God's Secretaries, 2003

  2. Though you could see all three places in one day, each of them makes you want to stay or to keep returning to watch the effects of the changing weather and the mercurial Sicilian light. —Francine Prose, Atlantic, December 2002

  3. Some scientists suggest that because manic-depressive patients are ever riding the bio-chemical express between emotional extremes, their brains end up more complexly wired and remain more persistently plastic than do the brains of less mercurial sorts. —Natalie Angier, New York Times, 12 Oct. 1993

  4. Still grinning, still miming, he samba-ed across the floor … and started in on the dishes with a vigor that would have prostrated his mercurial cousin. —T. Coraghessan Boyle, Harper's, October 1987

  5. <the boss's mood is so mercurial that we never know how he's going to react to anything>

Did You Know?

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."

Origin of mercurial

(see mercury)

First Known Use: 14th century

Synonym Discussion of mercurial

inconstant, fickle, capricious, mercurial, unstable mean lacking firmness or steadiness (as in purpose or devotion). inconstant implies an incapacity for steadiness and an inherent tendency to change <an inconstant friend>. fickle suggests unreliability because of perverse changeability and incapacity for steadfastness <performers discover how fickle fans can be>. capricious suggests motivation by sudden whim or fancy and stresses unpredictability <an utterly capricious critic>. mercurial implies a rapid changeability in mood <made anxious by her boss's mercurial temperament>. unstable implies an incapacity for remaining in a fixed position or steady course and applies especially to a lack of emotional balance <too unstable to hold a job>.

Rhymes with mercurial



noun mer·cu·ri·al \(ˌ)mər-ˈkyu̇r-ē-əl\

Definition of mercurial

  1. :  a pharmaceutical or chemical containing mercury

Origin of mercurial

(see mercury)

First Known Use: 1676

Other Chemical Engineering Terms

Seen and Heard

What made you want to look up mercurial? Please tell us where you read or heard it (including the quote, if possible).


marked by propriety and good taste

Get Word of the Day daily email!


Take a 3-minute break and test your skills!


Which of these is a synonym of unctuous?

maudlin oily prompt angry
Name That Thing

Test your visual vocabulary with our 10-question challenge!


Test Your Knowledge - and learn some interesting things along the way.