mantra vs underword

mantra

noun
  • A slogan or phrase often repeated. 

  • The hymn portions of the Vedas; any passage of these used as a prayer. 

  • A phrase repeated to assist concentration during meditation. 

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 mantra and underword occurred in a corpus of books? (source: Google Ngram Viewer )