adj. Volatile; erratic; unstable; flighty; fickle or changeable in temperament.adj. Lively; clever; sprightly; animated; quick-witted.adj. Of, or pertaining to the element mercury; containing mercury; caused by the action of mercury or quicksilver.adj. Having characteristics attributed to the Roman god Mercury, such as swiftness, eloquence, shrewdness, and thievishness.adj. (often capitalized, see Mercurial) Of or pertaining to the Roman god Mercury.adj. (often capitalized, see Mercurial) Of or pertaining to the planet Mercury; under the supposed astrological influence of the planet Mercury.n. A chemical containing mercury.