mer·cu·ri·al 
(mər-kyrē-əl)
(mər-kyrē-əl)adj.
1. often Mercurial
a. Roman Mythology Of or relating to the god Mercury.
b. 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.
n.
A pharmacological or chemical preparation containing mercury.
[Middle English, of the planet Mercury, from Latin mercuriālis, of the god or planet Mercury, from Mercurius, Mercury.]
mer·curi·al·ly adv.