quiet down vs retard

quiet down

verb
  • To diminish in intensity. 

  • To make someone or something become quieter. 

  • To become quieter. 

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 quiet down and retard occurred in a corpus of books? (source: Google Ngram Viewer )