let-down vs unhappy

let-down

noun
  • A disappointment or anticlimax. 

  • The clearance of an aircraft to descend through clouds to clear air below, or the process of granting such a clearance. 

  • The neurohormonal release of milk in dairy cows or in breastfeeding human mothers. 

unhappy

noun
  • An individual who is not happy. 

adj
  • Not happy; sad. 

  • Not satisfied; unsatisfied. 

How often have the words let-down and unhappy occurred in a corpus of books? (source: Google Ngram Viewer )