expound vs lessen

expound

verb
  • To set out the meaning of; to explain or discuss at length 

  • To make a statement, especially at length. 

lessen

verb
  • To make less; to diminish; to reduce. 

  • To become less. 

conj
  • unless. 

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