Musar על ביצה 50:27

"his spittle drips" — Because a foul spirit reposes upon him, and the power of anger and audacity implanted in him intensifies itself within him [(Beitzah 25b): "Three are audacious. The most audacious of the animals is the dog"], [he wants] to swallow anything that he chances upon, wherefore his mouth [always] drips spittle, especially so, in that his mouth is [always] open, and there is no restraint for it. This is the way of the confirmed speaker of lashon hara — to vent his anger at whoever he wants to.