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mercurial

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.

Definitions of mercurial
  1. adjective
    liable to sudden unpredictable change
    mercurial twists of temperament”
    synonyms: erratic, fickle, quicksilver
    changeable, changeful
    such that alteration is possible; having a marked tendency to change
  2. adjective
    relating to or containing or caused by mercury
    mercurial preparations”
    mercurial sore mouth”
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