When a person is described as mercurial, it implies that their emotional state or reactions can shift quickly and dramatically, often without apparent cause. This term conveys a sense of volatility and unpredictability, reflecting the idea that the person's disposition can change as swiftly as the element mercury.

a:changing moods quickly and often — used to describe someone who often changes from being happy to being angry or upset in a quick and unexpected way a mercurial movie star She had a mercurial personality/disposition.