initials vs pet name

initials

noun
  • An abbreviation of a person's name, constructed by taking the initial letters of first, last, and sometimes middle name(s). See monogram 

pet name

noun
  • A nickname, especially a name used by those in love with one another. 

  • Used other than figuratively or idiomatically: see pet, name. 

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