butter paper vs sheepskin

butter paper

noun

  • wax paper 

sheepskin

noun

  • The skin of a sheep, especially when used to make parchment or in bookbinding. 

  • A diploma. 

  • The tanned skin of a sheep with the fleece left on, especially when used for clothing, rugs, etc. 

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