idiom vs watchword

idiom

noun
  • An established phrasal expression whose meaning may not be deducible from the literal meanings of its component words. 

  • An artistic style (for example, in art, architecture, or music); an instance of such a style. 

  • A manner of speaking, a mode of expression peculiar to a language, language family, or group of people. 

  • A programming construct or phraseology that is characteristic of the language. 

  • A language or language variety; specifically, a restricted dialect used in a given historical period, context etc. 

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 idiom and watchword occurred in a corpus of books? (source: Google Ngram Viewer )