back-alley vs undercover

back-alley

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

undercover

adj
  • Performed or happening in secret. 

  • Employed or engaged in spying or secret investigation. 

noun
  • A person who works undercover. 

verb
  • To provide too little coverage. 

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