backstair vs covert

backstair

adj
  • clandestine, secret or furtive 

noun
  • The backstairs (stairs at the rear of a building). 

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 backstair and covert occurred in a corpus of books? (source: Google Ngram Viewer )