have a break vs slack off

have a break

verb
  • To have a short rest period from work, study, etc. 

slack off

verb
  • To decrease in intensity; to ease off; to diminish; to die down. 

  • To be deliberately unproductive in one's work or study. 

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