A level vs butt-fucking

A level

noun
  • anal sex. 

  • A non-compulsory examination taken in the final two years in British high schools; the qualification obtained by passing such an exam 

butt-fucking

noun
  • anal sex 

adj
  • Engaging in anal sex 

How often have the words A level and butt-fucking occurred in a corpus of books? (source: Google Ngram Viewer )