back-alley vs covert

back-alley

adj
  • Occurring away from publicity, or in mean surroundings; backstreet. 

covert

adj
  • Secret, surreptitious, concealed. 

  • Under coverture. 

noun
  • A covering. 

  • A disguise. 

  • A feather that covers the bases of flight feathers. 

  • Area of thick undergrowth where animals hide. 

  • A hiding place. 

How often have the words back-alley and covert occurred in a corpus of books? (source: Google Ngram Viewer )