co-father vs grandfather

co-father

noun
  • In a male homosexual couple, the nonbiological father (partner of the biological father) of a child. 

grandfather

noun
  • A father of someone's parent. 

  • A male forefather. 

  • The archived older version of a file that immediately preceded the father file. 

verb
  • To retain discontinued laws or rules for (a thing, person or organization previously affected by them). 

  • To be, or act as, a grandfather to. 

How often have the words co-father and grandfather occurred in a corpus of books? (source: Google Ngram Viewer )