co-wife vs old woman

co-wife

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

old woman

noun
  • One's wife. 

  • One's mother. 

  • An elderly woman. 

  • A person (irrespective of age or sex) who is always complaining about his or her situation. 

  • The plant sea wormwood. 

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