co-wife vs girlfriend

co-wife

noun
  • In a polygamous marriage, another wife of a woman’s husband. 

girlfriend

noun
  • A female partner in an unmarried romantic relationship. 

  • A fellow gay man, especially a friend as opposed to a sexual partner. 

  • A female friend. 

  • A term of address for a female friend or among gay men. 

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