mercurial Definition

1. Relating to or containing mercury.
2. Said of someone or their personality, mood, etc: lively or active; tending to change suddenly and unpredictably.

Thesaurus: changeable, erratic, fluctuating, inconstant, moody, capricious, fickle, flighty, unpredictable, volatile, impetuous, temperamental; Antonym: steady, stable, stolid.
3. Belonging or relating to either the god or the planet Mercury.