birthday suit vs naturism

birthday suit

noun
  • Nakedness, unclothed skin; an exposure of a human's natural anatomy without clothing. 

  • Used other than figuratively or idiomatically: see birthday, suit. 

naturism

noun
  • The belief in or practice of going nude in social settings, often in mixed-gender groups, specifically either in cultures where this is not the norm or for health reasons. 

  • The worship of the powers of nature. 

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