back forty vs back o' Bourke

back forty

noun

  • The most remote or inaccessible part of any place. 

  • The remote part of a farm. 

back o' Bourke

adv

  • At or to an extremely remote place. 

name

  • An extremely remote place. 

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