Timbuktu vs back forty

Timbuktu

name
  • Any proverbially distant or remote place. 

  • A city in central Mali. 

back forty

noun
  • The most remote or inaccessible part of any place. 

  • The remote part of a farm. 

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