Mercury is named after the planet Mercury. The planet is named after the Greek messenger god, representing speed. Mercury, being the only metal that's liquid at room temperature, has the nickname "quicksilver" because of its ability to flow. It's original name was "hydragyrum", which means "liquid silver." Its chemical symbol Hg comes from that name.
Mercury has been known since ancient times. It's been used in thermometers because of its consistent thermal expansion and in barometers because of its high density for a liquid, but it's been mostly replaced by other substances in those applications because of its toxicity. Mercury can enter the body by ingestion, inhalation, or through the skin, where it builds up in the tissues and can damage the brain, liver and kidneys. "Mad Hatters Disease" was a name given to the mental degradation experienced by some people who wore felt hats that had been treated with mercury to maintain their shape.
Mercury is a good electrical conductor and is useful for switches in devices that are activated or deactivated by tilting, such as space heaters that turn off when tipped over. The mercury moves and breaks an electrical connection when the container it's in tilts.