mercurial

/(ˌ)mər-ˈkyu̇r-ē-əl/

adjective

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

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

  3. characterized by rapid and unpredictable changeableness of mood

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. fickle suggests unreliability because of perverse changeability and incapacity for steadfastness.

noun

  1. a pharmaceutical or chemical containing mercury