drop off vs obtund

drop off

verb
  • To lessen or reduce. 

  • To fall. 

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

  • To fall asleep. 

obtund

verb
  • To reduce the edge or effects of; to mitigate; to dull. 

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