mercurial vs undependable

mercurial

noun
  • A person born under the influence of the planet Mercury; hence, a person having an animated, lively, quick-witted or volatile character. 

  • A chemical compound containing mercury. 

  • A preparation of mercury, especially as a treatment for syphilis. 

adj
  • Of or pertaining to the element mercury or quicksilver; containing mercury. 

  • Caused by the action of mercury or a mercury compound. 

  • Pertaining to the astrological influence of the planet Mercury; having the characteristics of a person under such influence (see adjective sense 1). 

  • Pertaining to Mercury, the Roman god of, among other things, commerce, financial gain, communication, and thieves and trickery; hence (comparable), money-making; crafty. 

  • Pertaining to the planet Mercury. 

  • Having a lively or volatile character; animated, changeable, quick-witted. 

undependable

adj
  • Not dependable. 

How often have the words mercurial and undependable occurred in a corpus of books? (source: Google Ngram Viewer )