ennui vs sadness

ennui

noun
  • A gripping listlessness or melancholia caused by boredom; depression. 

verb
  • To make bored or listless; to weary. 

sadness

noun
  • An event in one's life that causes sadness. 

  • The state or emotion of being sad. 

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