indigestible vs iron-hearted

indigestible

adj

  • Difficult to accept; unpalatable. 

  • Difficult or impossible to digest. 

noun

  • Anything that is difficult to digest. 

iron-hearted

adj

  • pitiless; inflexibly unsympathetic 

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