middle name vs watchword

middle name

noun
  • A trait, activity, or action closely associated with or aptly describing a person. 

  • A name between the first given name and the family name or surname; a second or subsequent given name. 

  • One's mother's maiden name. 

watchword

noun
  • A word used as a motto, as expressive of a principle, belief, or rule of action; a rallying cry. 

  • A prearranged reply to the challenge of a sentry or a guard; a password or signal by which friends can be known from enemies. 

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