gag vs keep back

gag

verb
  • To restrain someone's speech without using physical means. 

  • To restrain someone's speech by blocking his or her mouth. 

  • To cause to heave with nausea. 

  • To pry or hold open by means of a gag. 

  • To choke; to retch. 

  • To experience the vomiting reflex. 

  • To astonish (someone); to be at a loss for words. 

noun
  • A joke or other mischievous prank. 

  • Mycteroperca microlepis, a species of grouper. 

  • Any suppression of freedom of speech. 

  • An order or rule forbidding discussion of a case or subject. 

  • A device to restrain speech, such as a rag in the mouth secured with tape or a rubber ball threaded onto a cord or strap. 

  • a device or trick used to create a practical effect; a gimmick 

  • A convulsion of the upper digestive tract. 

keep back

verb
  • To restrict or restrain. 

  • To hold back; to refuse to give or share. 

  • To prevent (a pupil) from advancing in a course. 

  • To stay back; to refrain from approaching. 

How often have the words gag and keep back occurred in a corpus of books? (source: Google Ngram Viewer )