What does mercurial mean?
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.
Above retrieved from Answers.com
It means being quick and changeable in temperament; volatile, fickle, flighty
or - more archaically -
being eloquent, shrewd, fast, and "thievish" like the Roman god Mercury