You can speak any adverb. You can speak loudly, softly, gradually, democratically and deliciously. You can speak a few nouns: English and the truth. Or you could speak words as sharp as daggers, or as cruel. But you can’t speak daggers any more than you can speak grenades or bullets or blunderbusses. And that’s why the phrase stuck. Speaking daggers is so unusual that it became part of the language. And then it became usual.
