deboard vs slip off

deboard

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

slip off

verb
  • To leave a port, anchorage or mooring 

  • To leave a place, or a meeting, without being noticed 

  • To remove an article of clothing 

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