In Karate Way, often I've discussed the many Japanese idioms and sayings that refer to the sword. This aspect of colloquial Japanese reminds one of how deeply the sword and the warrior influenced the culture of that country.
Thinking about these figures of speech, I remembered one that I heard as a child: umi no uchi no katana, "the sword behind the smile." This is a curious saying. How should one interpret it? A smile behind the sword would seem obvious in meaning. You are ready, even eager to use the weapon and happy to do so. But the other way around? We associate smiles with politeness and friendliness. The sword hiding behind that seems incompatible.