Here's the breakdown:
* Mercury was the Roman god of commerce, travel, communication, and thievery. He was known for his swiftness and cunning.
* The planet Mercury, named after the god, was also associated with speed and variability due to its rapid movement across the sky.
* This association with speed and variability led to the adjective "mercurial" being used to describe something that is changeable, volatile, and unpredictable.
Therefore, the word "mercurial" traces its roots back to the ancient Roman mythology and the qualities attributed to the god Mercury.