/ˌmɜːrkəˈriəl/ MAHR-kə-ree-əl

Definitions

1. adjective

having a tendency to change frequently or unpredictably; unreliable or unstable

“His mercurial temper made it difficult to work with him.”

2. noun

a person whose moods or temper change quickly and unpredictably

“He’s known to be a bit of a mercurial in the office.”