dropping vs out of gas

dropping

noun
  • The act of something that drops or falls. 

  • Something dropped; a droplet. 

  • A piece of animal excrement; dung. 

out of gas

prep
  • Tired; lacking energy or motivation. 

  • Used other than figuratively or idiomatically: see out of, gas.: lacking fuel. 

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