let-down vs mournful

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. 

mournful

adj
  • Fit to inspire mourning; tragic. 

  • Filled with grief or sadness; being in a state in which one mourns. 

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