What is the meaning of the word "quick silver"?

Definitions:

  1. liable to sudden unpredictable change copy to clipboard adjective
    • If you want to stay in one place, don't ask the children where they want to go; they are quicksilver. copy to clipboard
  2. liquid mercury copy to clipboard noun
    • The beaker broke and quicksilver ran across the lab station and beaded on the floor. copy to clipboard

Synonyms: