slackness vs thoughtlessness

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. 

thoughtlessness

noun
  • heedless disregard for the consequences of one's actions 

  • disregard for other's feelings 

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