lassitude vs slackness

lassitude

noun
  • Listlessness or languor. 

  • Lethargy or lack of energy; fatigue. 

slackness

noun
  • The state of being slack; the quality of having slack. 

  • A subgenre of dancehall music with sexual lyrics. 

  • Vulgarity in West Indian culture. 

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