disuse vs let down

disuse

verb
  • To cease the use of. 

noun
  • The state of not being used; neglect. 

let down

verb
  • To reduce one's level of effort. 

  • To disappoint; to betray or fail somebody 

  • To soften in tempering. 

  • Used other than figuratively or idiomatically: see let, down. 

  • To allow to descend. 

  • To lengthen by undoing and resewing a hem. 

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