Mercurial describes someone whose mood or behavior is changeable and unpredictable, or someone who is clever, lively, and quick. With a mercurial teacher, you never know where you stand.
Mercury was the ancient Roman god of commerce and messenger of the gods, and the planet Mercury was named after the Roman god. In Middle English, this adjective meant "relating to the planet or god Mercury" and derives from Latin mercuriālis, from Mercurius "Mercury." A mercurial personality has the unpredictability associated with the god Mercury or, in astrology, is supposedly influenced by the planet.