Humble, Discreet
You say something to a boot when you want to be humble. You're down there because you're bowing in front of the person you're humbling yourself before. Because of that, your face is really close to their boots and you're speaking directly at them, instead of at the person's face, so it's more discreet.
Picture yourself trying to be humble. You're down on the ground talking to a pair of boots. You don't dare raise your eyes past that.
きん
つつし
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You're being humble like this because those boots belong to the King (きん) of England. You can't just say whatever you want at eye level to a king. He'll have your head chopped off for insolence, or worse!
Imagine saying something to the King. Get low down on the ground to be as humble as you can. Whisper into the King's boots. Lick them if you have to.