cast iron vs involuntary

cast iron

adj
  • Inflexible or without exception. 

  • Made of cast iron. 

  • Durable; tough; resilient. 

noun
  • A hard and brittle, but strong, alloy of iron, carbon, and silicon, formed by casting in a mould. 

involuntary

adj
  • Without intention; unintentional. 

  • Not voluntary or willing; contrary or opposed to explicit will or desire; unwilling. 

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