backset vs retroversion

backset

noun
  • A check; a relapse; a discouragement; a setback. 

  • Whatever is thrown back in its course, such as water. 

verb
  • To plow again in the fall; said of prairie land broken up in the spring. 

retroversion

noun
  • The state or condition of being retroverted. 

  • A turning or falling back. 

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