“The tongue like a sharp knife… Kills without drawing blood.”
It is said that a dull knife is far more dangerous than a sharp one. The idea being that a dull jagged blade will cut with tearing and great mess, whereas a sharp edge will cut with surgical precision and no tearing. Using this metaphor with the tongue, which of course forms our words, sends a powerful message. It is saying that words can be very powerful, and can do great damage, can even kill, but without drawing actual blood