Hg is the modern chemical symbol for mercury.
It comes from hydrargyrum, a Latinized form of the Greek word `Υδραργυρος (hydrargyros), which is a compound word meaning 'water' and 'silver' — since it is liquid, like water, and yet has a silvery metallic sheen.
The element was named after the Roman god Mercury, known for speed and mobility. It is associated with the planet Mercury. The astrological symbol for the planet is also one of the alchemical symbols for the metal.
Mercury is the only metal for which the alchemical planetary name became the common name.
From Pocket Wikipedia, http://www.free-soft.ro/pocket-wikipedia/