billfold vs bursa

billfold

noun
  • A small, folding sleeve or case designed to hold paper currency, as well as credit cards, pictures, etc. 

bursa

noun
  • Any of the many small fluid-filled sacs located at the point where a muscle or tendon slides across bone. These sacs serve to reduce friction between the two moving surfaces. 

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