Source : Webster's Revised Unabridged Dictionary (1913)

Mercurial \Mer*cu"ri*al\, n.
   1. A person having mercurial qualities. --Bacon.

   2. (Med.) A preparation containing mercury.

Mercurial \Mer*cu"ri*al\, a. [L. mercurialis, fr. Mercurius
   Mercury: cf. F. mercuriel.]
   1. Having the qualities fabled to belong to the god Mercury;
      swift; active; sprightly; fickle; volatile; changeable;
      as, a mercurial youth; a mercurial temperament.

            A mercurial man Who fluttered over all things like a
            fan.                                  --Byron.

   2. Having the form or image of Mercury; -- applied to ancient
      guideposts. [Obs.] --Chillingworth.

   3. Of or pertaining to Mercury as the god of trade; hence,
      money-making; crafty.

            The mercurial wand of commerce.       --J. Q. Adams.

   4. Of or pertaining to, or containing, mercury; as, mercurial
      preparations, barometer. See {Mercury}, 2.

   5. (Med.) Caused by the use of mercury; as, mercurial sore
      mouth.

Source : WordNet®

mercurial
     adj 1: liable to sudden unpredictable change; "erratic behavior";
            "fickle weather"; "mercurial twists of temperament";
            "a quicksilver character, cool and willful at one
            moment, utterly fragile the next" [syn: {erratic}, {fickle},
             {quicksilver(a)}]
     2: relating to or under the (astrological) influence of the
        planet Mercury; "the Mercurial canals"
     3: relating to or having characteristics (eloquence,
        shrewdness, swiftness, thievishness) attributed to the god
        Mercury; "more than Mercurial thievishness"
     4: relating to or containing or caused by mercury; "mercurial
        preparations"; "mercurial sore mouth"