Bumfuck vs back o' Bourke

Bumfuck

name

  • An imaginary place in the middle of nowhere. 

back o' Bourke

name

  • An extremely remote place. 

adv

  • At or to an extremely remote place. 

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