backstair vs nonpublic

backstair

adj

  • clandestine, secret or furtive 

noun

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

nonpublic

adj

  • Not open to the public; not available to or shared by the public; private. 

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