1.Definition: liable to sudden unpredictable change

"Mercurial twists of temperament"

Related Adjective(s):erratic, fickle, quicksilver

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"