X-rated vs XXX

X-rated

adj
  • Pornographic or indecent. 

  • Denoting a film given an X classification. 

XXX

adj
  • Designating extreme or hardcore pornography. 

  • Extra strong; very high quality. 

verb
  • To delete; to cross out, especially to conceal or suggest vulgar language. 

noun
  • Triple X syndrome. 

symbol
  • An indicator of hazard or poison. 

  • Traditional brewer's mark for barrels of extra strong beer—extended to other alcoholic beverages and other products having an extra strong dimension (varying from laundry soap to pornography). 

How often have the words X-rated and XXX occurred in a corpus of books? (source: Google Ngram Viewer )