co-wife vs old lady

co-wife

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

old lady

noun
  • One's girlfriend, wife or significant other. 

  • The Bank of England, sometimes referred to as the “Old Lady of Threadneedle Street” or simply “The Old Lady”. 

  • A queen. 

  • One's mother. 

  • An elderly woman. 

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