catchword vs underword

catchword

noun
  • A word or expression repeated until it becomes representative of a party, school, business, or point of view. 

  • The last word of a speech, serving as a cue for the next speaker. 

  • A word under the right-hand side of the last line on a book page that repeats the first word on the following page. 

underword

noun
  • A word placed below another, either literally or metaphorically: a subtext; an inadequate or subordinate term or description; a secret word or phrase; a watchword 

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