deboard vs drop off

deboard

verb
  • To exit a form of transportation such as a boat, ship, airplane, trolley, streetcar or spaceship. 

drop off

verb
  • To deliver; to deposit or leave; to allow passengers to alight. 

  • To fall. 

  • To lessen or reduce. 

  • To fall asleep. 

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