retard vs work on

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. 

work on

verb
  • To exercise influence on someone. 

  • To shape, form or improve something. 

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