liable vs unimpregnable

liable

adj
  • subject; susceptible. 

  • exposed to a certain contingency or causality, more or less probable. 

  • bound or obliged in law or equity; responsible; answerable. 

  • likely. 

unimpregnable

adj
  • Incapable of being impregnated; impermeable or infertile. 

  • Not impregnable; capable of being pierced or assailed. 

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