What does ‘mercurial’ mean?

The Romans named many of the planets in our solar system in honour of their gods - Venus, Jupiter, Mars, Mercury, etc. The word ‘mercurial’ comes from Mercury, and in Roman mythology, Mercury was the messenger of the gods. His job required speed; and the winged sandals and the winged helmet he wore enabled him to get from one place to another very quickly. Speed was one of the reasons why the small planet was named Mercury. Like the god, the planet closest to the sun is very quick as well. Among the planets, Mercury is the fastest to orbit the sun - while it takes Earth 365 days to go around the sun, Mercury takes a mere 88 days.