heard this word yesterday... did not know what it meant

mer·cu·ri·al
mərˌkyo͝orēəl/
adjective
  1. 1.
    (of a person) subject to sudden or unpredictable changes of mood or mind.
    "his mercurial temperament"
  2. 2.
    of or containing the element mercury.
noun
  1. 1.
    a drug or other compound containing mercury.