co-mother vs old lady

co-mother

noun
  • In a lesbian couple, the nonbirth mother (partner of the birth mother) of a child, especially one who takes an equal role in mothering the child. 

  • In polygamy, the relationship between one co-wife and another, in respect to their children. 

  • In polygamy, a wife of one's father who is not one's mother; that is, co-wife of one's mother. 

old lady

noun
  • One's mother. 

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

  • A queen. 

  • An elderly woman. 

  • One's girlfriend, wife or significant other. 

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