batten down vs unfasten

batten down

verb
  • To close or make watertight, referring to hatches and cargo. 

  • To secure. 

  • To prepare for adversity. 

unfasten

verb
  • To come unloosed or untied. 

  • To detach from any connecting agency or link; to disconnect. 

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