let in on vs retard

let in on

verb
  • To disclose information to someone; to tell somebody a secret or share privileged information. 

retard

verb
  • To put off; to postpone. 

  • To decelerate; to slow down. 

  • 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 let in on and retard occurred in a corpus of books? (source: Google Ngram Viewer )