1 - often Mercurial
1a - Roman Mythology. Of or relating to the god Mercury.
1b - Astronomy. Of or relating to the planet Mercury.
2 - Having the characteristics of eloquence, shrewdness, swiftness, and thievishness attributed to the god Mercury.
3 - Containing or caused by the action of the element mercury.
4 - Quick and changeable in temperament; volatile: a mercurial nature.