ennui vs lassitude

ennui

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

verb
  • To make bored or listless; to weary. 

lassitude

noun
  • Listlessness or languor. 

  • Lethargy or lack of energy; fatigue. 

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