MERCURIAL = (adjective) 1. containing or being somehow related to mercury; 2. having a temperamental, active, or unpredictable personality; 3. quick, lively, and shrewd, like the god Mercury Synonyms: erratic, changeable, fickle, volatile, unpredictable, fluctuating, temperamental, moody, lively, quick Antonyms: constant, invariable, steady, stable, even Examples: - I wanted a mercurial thermometer but I got <...>