cave-in vs drop-off

cave-in

drop-off

noun
  • A sudden downward slope. 

  • A delivery; the act of leaving a package, etc. 

  • A visitor to a website who ceases to continue using and navigating around the site after reaching some specified page; or such an act of cessation. 

  • The precipitous outer side of a coral reef, facing the open sea. 

  • A sudden decrease. 

  • A space reserved outside a bus or railway station for vehicles stopping to drop off passengers for onward transit. 

  • A time during which passengers, such as school children, are dropped off. 

How often have the words cave-in and drop-off occurred in a corpus of books? (source: Google Ngram Viewer )