Etymology
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mercurial (adj.)

late 14c., "pertaining to or under the influence of the planet Mercury," from Latin Mercurialis, from Mercurius (see Mercury). Meaning "pertaining to the god Mercury, having the form or qualities attributed to Mercury" (in reference to his role as god of trade or as herald and guide) is from 1590s. Meaning "light-hearted, sprightly, volatile, changeable, quick" (1640s) is from the qualities supposed to characterize those born under the planet Mercury (they also are the qualities of the god Mercury), probably also partly by association with the qualities of quicksilver. A variant in this sense was mercurious (1590s). Related: Mercurially; mercuriality.

updated on December 29, 2018

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Definitions of mercurial from WordNet
1
mercurial (adj.)
liable to sudden unpredictable change;
mercurial twists of temperament
Synonyms: erratic / fickle / quicksilver
mercurial (adj.)
relating to or containing or caused by mercury;
mercurial sore mouth
mercurial preparations
2
Mercurial (adj.)
relating to or under the (astrological) influence of the planet Mercury;
Mercurial (adj.)
relating to or having characteristics (eloquence, shrewdness, swiftness, thievishness) attributed to the god Mercury;
Etymologies are not definitions. From wordnet.princeton.edu, not affiliated with etymonline.