Definition of mercurial

• 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"
• relating to or under the (astrological) influence of the planet Mercury; "the Mercurial canals"
• relating to or having characteristics (eloquence, shrewdness, swiftness, thievishness) attributed to the god Mercury; "more than Mercurial thievishness"