retard vs turn back

retard

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

  • To put off; to postpone. 

  • To decelerate; to slow down. 

noun
  • Retardation; delay. 

  • A slowing down of the tempo; a ritardando. 

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

turn back

verb
  • To prevent or refuse to allow passage or progress. 

  • To cause to reverse direction and retrace one's steps. 

  • To return to a previous state of being. 

  • To reverse direction and retrace one's steps. 

  • To adjust to a previous setting. 

  • To fold something back; to fold down. 

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