impregnable vs vulnerable

impregnable

adj
  • Capable of being impregnated; impregnatable. 

  • Of a fortress or other fortified place: able to withstand all attacks; impenetrable, inconquerable, unvanquishable. 

  • Too strong to be defeated or overcome; invincible. 

vulnerable

adj
  • More or most likely to be exposed to the chance of being attacked or harmed, either physically or emotionally. 

  • Open to disclosing one's inner thoughts and feelings, acting in spite of one's instinct to self-preservation. 

  • More likely to be exposed to malicious programs or viruses. 

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