definition of
Mercurial

[1] #1 Mercurial s. [erratic, fickle, mercurial, quicksilver] 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". wn

[2] #2 Mercurial a. relating to or under the (astrological) influence of the planet Mercury; "the Mercurial canals". wn

[3] #3 Mercurial a. relating to or having characteristics (eloquence, shrewdness, swiftness, thievishness) attributed to the god Mercury; "more than Mercurial thievishness". wn

[4] #4 Mercurial a. [mercurial] relating to or containing or caused by mercury; "mercurial preparations"; "mercurial sore mouth". wn