burro vs duffer

burro

noun
  • A small donkey, especially when used as a pack animal or one that is feral and lives in the southwestern United States or northern Mexico. 

duffer

noun
  • A cow that does not produce milk. 

  • An incompetent, indolent, or clumsy person. 

  • Any of various nymphalid butterflies of the genus Discophora. 

  • A player having little skill, especially a golfer who duffs. 

  • A racing pigeon that does not perform well. 

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