boon vs cave-in

boon

adj
  • gay; merry; jovial; convivial. 

noun
  • The woody portion of flax, separated from the fiber as refuse matter by retting, braking, and scutching. 

  • A good thing; a blessing or benefit; a thing to be thankful for. 

  • An unpaid service due by a tenant to his lord. 

cave-in

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