noun

1. Any of the plants known as mercury, especially the annual mercury or French mercury (Mercurialis annua).

2. A person born under the influence of the planet Mercury; hence, a person having an animated, lively, quick-witted or volatile character.

3. A chemical compound containing mercury.

adjective

1. Having a lively or volatile character; animated, changeable, quick-witted.

   “his mercurial temperament”

2. Pertaining to the astrological influence of the planet Mercury; having the characteristics of a person under such influence (see adjective sense 1).

3. Pertaining to the planet Mercury.

Synonyms: fickle, unpredictable