lassitude vs melancholy

lassitude

noun

  • Lethargy or lack of energy; fatigue. 

  • Listlessness or languor. 

melancholy

noun

  • Great sadness or depression, especially of a thoughtful or introspective nature. 

  • Black bile, formerly thought to be one of the four "cardinal humours" of animal bodies. 

adj

  • Affected with great sadness or depression. 

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