anus vs back passage

anus

noun
  • The lower orifice of the alimentary canal, through which feces and flatus are ejected. 

  • An impolite, unpopular, or unintelligent person. 

back passage

noun
  • anus, rectum 

  • The passage between two terraced houses leading between the front and back gardens or yards. 

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