1.Definition: liable to sudden unpredictable change
"Mercurial twists of temperament"
2.Definition: relating to or containing or caused by mercury
"Mercurial preparations", "Mercurial sore mouth"
3.Definition: relating to or having characteristics (eloquence, shrewdness, swiftness, thievishness) attributed to the god Mercury
"More than Mercurial thievishness"
4.Definition: relating to or under the (astrological) influence of the planet Mercury
"The Mercurial canals"