What is the meaning of quicksilver?

noun

• A heavy silvery toxic univalent and bivalent metallic element; the only metal that is liquid at ordinary temperatures.
  • synonyms: mercury, hydrargyrum, Hg, atomic number 80

adjective

• Liable to sudden unpredictable change.
  • usage: "erratic behavior"; "fickle weather"; "mercurial twists of temperament"; "a quicksilver character, cool and willful at one moment, utterly fragile the next"
  • synonyms: erratic, fickle, mercurial