The phrase "down in mouth" refers to someone who is sad or depressed. There are several antonyms for this expression, including "happy," "joyful," "cheerful," "optimistic," "content," and "upbeat." If someone is not "down in the mouth," they may instead be "smiling," "laughing," or "grinning." They may have a positive outlook on life, be feeling satisfied with their circumstances or experiencing moments of joy. While feeling down in the mouth is a natural part of life at times, it's important to seek out sources of joy and hope to counteract negative feelings and improve our mental health.