indigestible vs tough as nails

indigestible

adj

  • Difficult or impossible to digest. 

  • Difficult to accept; unpalatable. 

noun

  • Anything that is difficult to digest. 

tough as nails

adj

  • Very hard; tough 

  • Having a hard, strong, and determined mindset. 

How often have the words indigestible and tough as nails occurred in a corpus of books? (source: Google Ngram Viewer )