Having rapid and unpredictable changes of mood.
Example:
His mother's always mercurial temper became even more unpredictable, to the point where the slightest thing would trigger a violent fit.
The god Mercury and the planet named for him were thought to govern eloquence and cleverness. As the gods' messenger, with his winged cap and sandals, he was the very symbol of speed. The planet Mercury was named for him because it is the fastest of the planets. His name was also given to the liquid silver metal that skitters out of one's hand so quickly it is almost impossible to hold. A mercurial person isn't necessarily physically quick, but changes moods with bewildering speed.