mer·cu·ri·al [mer-kyoor-ee-uhl] (from Dictionary.com)
1. changeable; volatile; fickle; flighty; erratic: a mercurial nature.
2. animated; lively; sprightly; quick-witted.
3. pertaining to, containing, or caused by the metal mercury.
4. (initial capital letter) of or pertaining to the god Mercury.
5. (initial capital letter) of or pertaining to the planet Mercury.
6. Pharmacology. a preparation of mercury used as a drug.
[Origin: 1350–1400; ME < L mercuriālis of, pertaining to the god or planet Mercury. See mercury, -al1]