The phrase "running at the mouth" means to talk excessively, especially in a forceful or angry way. Antonyms for this phrase include being quiet, silent, reserved, and reticent. These words describe someone who is not inclined to speak out or share their opinions freely. Another antonym for "running at the mouth" is to be concise, brief, and to the point. Instead of rambling on, being clear and succinct with your words can speak volumes without saying much at all. Ultimately, using these antonyms can help improve communication and avoid misunderstanding or offending others with excessive speech.