A "mug" in the context of a face refers to someone's facial expression or appearance, often used informally. When someone says, "Don't make that mug," they are usually asking the person to stop making a silly or exaggerated face. This term can also describe a person's overall look, especially if it’s unique or memorable. In a more playful sense, a mug can also refer to a person who is being silly or foolish. For example, if someone is acting goofy, you might say they are "mugging for the camera." This playful behavior often brings laughter and light-heartedness to social situations, making it a fun way to express oneself.