back out vs cop out

back out

verb
  • To dare (someone) to not withdraw from a challenge. 

  • To withdraw from something one has agreed to do. 

  • To exit a mode or function. 

  • To draw from behind the back [+accusative = a knife etc.] (as also bare back). 

  • To undo (a change). 

  • Coordinate terms: cancel, call off, get cold feet 

  • To reverse (a vehicle) from a confined space. 

  • See also: sell short 

  • To bet on someone losing. 

  • Coordinate term: scare off 

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 back out and cop out occurred in a corpus of books? (source: Google Ngram Viewer )