Mercury

"the Roman god Mercury," herald and ambassador of his father, Jupiter, mid-12c., Mercurie, from Latin Mercurius "Mercury," originally a god of tradesmen and thieves, from merx "merchandise" (see market (n.)); or perhaps [Klein, Tucker] from Etruscan and influenced by merx. De Vaan thinks it possible the whole stem *merk- was borrowed and the god-name with it. Mercury later was identified with Greek Hermes and still later with Germanic Woden. From his role as a messenger and conveyor of information, since mid-17c. Mercury has been a common name for a newspaper. 

The planet closest to the sun was so called in classical Latin (c. 1300 in English). A hypothetical inhabitant of the planet was a Mercurean (1855) or a Mercurian (1755). For the metallic element, see mercury.

silver-white fluid metallic element, late 14c., from Medieval Latin mercurius, from Latin Mercurius (see Mercury). Prepared in ancient times from cinnabar, it was one of the seven metals (bodies terrestrial) known to the ancients, which were coupled in astrology and alchemy with the seven known heavenly bodies. This one probably was associated with the planet for its mobility. The others were Sun/gold, Moon/silver, Mars/iron, Saturn/lead, Jupiter/tin, Venus/copper.

The Greek name for it was hydrargyros "liquid silver," which gives the element its symbol, Hg. Compare quicksilver, which is its popular name. It has a freezing point of -39° C. The use of the word in reference to temperature or state of the atmosphere (by 1756) is from its use in thermometers and barometers.