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1. Liable to sudden unpredictable change
   "mercurial twists of temperament";
   - erratic, fickle, quicksilver
    
2. Relating to, containing or caused by mercury
   "mercurial preparations"; "mercurial sore mouth"

1. Relating to or under the (astrological) influence of the planet Mercury
   "the Mercurial canals"
    
2. Relating to or having characteristics (eloquence, shrewdness, swiftness, thievishness) attributed to the god Mercury
   "more than Mercurial thievishness"

See also: changeable, changeful

Encyclopedia: Mercurial