Definition of Mercurialist in US English:

Mercurialist

noun

rare, historical
  • A person born under the influence of the planet Mercury; a person having the qualities attributed to such a birth, especially eloquence, ingenuity, or aptitude for trade. Also occasionally with unfavourable connotation: a sharper, a thief.

Origin

Mid 16th century; earliest use found in Thomas Drant (c1540–1578), poet and Church of England clergyman. From mercurial + -ist.

Pronunciation

Mercurialist

/məːˈkjʊərɪəlɪst/