forbidden fruit vs grapefruit

forbidden fruit

noun
  • A small variety of shaddock (Citrus maxima), or sometimes other citrus fruits. 

  • Illicit pleasure. 

  • The fruit forbidden to Adam. 

  • Something desired that cannot be had. 

grapefruit

noun
  • The tree of the species Citrus paradisi, a hybrid of pomelo (Citrus maxima) and sweet orange. 

  • The large spherical tart fruit produced by this tree. 

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