Here's the breakdown:
* Mercury was the Roman god of commerce, travel, and communication, known for his speed and agility.
* Mercury was also associated with the planet Mercury, which moves quickly across the sky.
* Mercurial therefore came to describe someone who is quick-witted, lively, and prone to sudden changes in mood or behavior. This reflects the swift and unpredictable nature associated with both the god and the planet.
So, the next time you encounter someone described as "mercurial," remember the Roman god of speed and agility, whose name gave rise to this descriptive word.