backstair vs under the table

backstair

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

adj
  • clandestine, secret or furtive 

under the table

prep
  • Used other than figuratively or idiomatically: see under, table. 

  • In or into a state of stupification or boredom. 

  • In or into a state of drunkenness. 

  • Secret; covert; undisclosed. 

  • Secretly; covertly; undisclosedly. 

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