1. quick and changeable in temperament; volatile 2. of or relating to the god (or planet) Mercury 3. having the characteristics of eloquence, shrewdness, swiftness, and thievishness attributed to the god Mercury

This word, or at least a variant of it, is perhaps used most famously in Shakespeare’s Romeo and Juliet, in the character Mercutio. More recently, Kate Atkinson uses the word in her novel Case Histories: She had a bewilderingly mercurial personality—punching and kicking one minute, a sham of cooing and kissing the next.