lover vs side piece

lover

noun
  • A sexual partner, especially one with whom someone is having an affair. 

  • A person who loves something. 

  • An informal term of address for any friend. 

  • One who loves and cares for another person in a romantic way; a sweetheart, love, soulmate, boyfriend, girlfriend, or spouse. 

side piece

noun
  • A mistress; a woman with whom a man carries on a physical affair (not emotional affair) without having the intention of forming a serious relationship. 

How often have the words lover and side piece occurred in a corpus of books? (source: Google Ngram Viewer )