bungle vs slack off

bungle

verb
  • To act or work incompetently; to fumble. 

  • To incompetently perform (a task); to ruin (something) through incompetent action; to botch up, to bumble. 

noun
  • A botched or incompetently handled action or situation; a blunder. 

slack off

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

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

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