libretto vs parts

libretto

noun
  • The text of a dramatic musical work, such as an opera. 

  • A book containing such a text. 

parts

noun
  • Intellectual ability or learning. 

  • Vicinity, region. 

  • The genitals, short for private parts. 

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