beg off vs cop out

beg off

verb
  • To obtain somebody's permission to skip or cancel an event that one has previously arranged with them. 

cop out

verb
  • To avoid or shirk, either by failing to perform, or by performing in a grossly insufficient, negligent, or superficial manner. 

  • To plead guilty and ask for mercy. 

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