Mercurial is an adjective that means characterized by rapid, unpredictable change.
Our word of the day shares its roots with a planet and a Roman god. Mercury was Rome’s equivalent to the Greek god Hermes, and was known for being eloquent and ingenious. When mercurial entered the English language in the 14th century, the word was used to describe someone possessing these qualities. In time its meaning shifted to ‘unpredictable’ and ‘changeable’ in reference to the chemical used in thermometers, known for its quick changes as it rises and falls to reflect the temperature.
I was hoping we could hire a basketball coach who was a little more predictable and stable than our previous one. Coach Derringer’s mercurial nature was a real detriment to the team.