lessen vs retard

lessen

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

  • To become less. 

conj
  • unless. 

retard

verb
  • To decelerate; to slow down. 

  • To put off; to postpone. 

  • To keep delaying; to continue to hinder; to prevent from progressing. 

noun
  • Retardation; delay. 

  • A slowing down of the tempo; a ritardando. 

  • A person or being who is extremely stupid or slow to learn. 

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