It comes from the Roman god Mercury, known for his swiftness, wit, and trickery.
Here are some examples of how "mercurial" can be used:
* "The stock market is notoriously mercurial, rising and falling in unpredictable ways."
* "He was a mercurial artist, constantly changing his style and subject matter."
* "Her mood was mercurial, shifting from joy to anger in the blink of an eye."
Essentially, "mercurial" implies a quick and fluctuating nature, often making it difficult to predict or understand.