bummed out vs let-down

bummed out

adj
  • Depressed, disappointed, in a gloomy mood. 

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. 

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