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Definition of mercurial : 1. A person having mercurial qualities.
2. A preparation containing mercury.
3. Caused by the use of mercury; as, mercurial sore mouth.
4. Having the form or image of Mercury; - applied to ancient guideposts.
5. Having the qualities fabled to belong to the god Mercury; swift; active; sprightly; fickle; volatile; changeable; as, a mercurial youth; a mercurial temperament.
6. Of or pertaining to, or containing, mercury; as, mercurial preparations, barometer. See Mercury, 2.

Synonyms:

quicksilver, unpredictable, fantastic, whimsical, irregular, erratic, capricious, temperamental, uncertain, fluctuating, inconsistent, fantastical, ticklish, change, continue, freakish

inconstant, unsteady, flexible, alterable, restless, adaptable, irresolute, unsettled, vacillating, modifiable, transposable, varying, precarious, movable, shifting, convertible, mutable, protean, wavering, variable, adjustable, fluid, changeable, reformable, unstable, vicissitudinous, fickle, changeful, volatile, versatile, flighty, transformable, ever-changing

Usage examples:

• And Fullaway, active and mercurial though he was, made no attempt at conversation- he sat with knitted forehead, trying to think, to account, to surmise, only conscious that he was up against a bigger mystery than life had ever shown him up to then. - "The Rayner-Slade Amalgamation", J. S. Fletcher.
• Poor Larry O'Hale could not thus calm his mercurial spirit. - "Sunk at Sea", R.M. Ballantyne.
• While he stormed on, saying things that he would willingly have taken back a minute later, every bit of its usual mercurial humor drained out of Billy's face. - "The Mystery of The Barranca", Herman Whitaker.