encumber vs retard

encumber

verb
  • To restrict or block something with a hindrance or impediment. 

  • To burden with a legal claim or other obligation. 

  • To load down something with a burden. 

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. 

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