aptitude vs unfitness

aptitude

noun
  • Natural ability to acquire knowledge or skill. 

  • The condition of being suitable. 

unfitness

noun
  • The characteristic of being unfit or out of physical shape. 

  • The characteristic of being unfit or ill-suited. 

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