locution vs underword

locution

noun
  • The use of a word or phrase in an unusual or specialized way. 

  • A phrase or expression connected to an individual or a group of individuals through repeated usage. 

  • A supernatural revelation where a religious figure, statue or icon speaks, usually to a saint. 

  • Style of discourse or usage, or any particular utterance in such style. 

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