"Am down the mouth" is a colloquial expression that means to feel sad or unhappy. There are several antonyms for this phrase, including "jovial," "happy-go-lucky," "merry," and "cheerful." These words all describe a person who is feeling upbeat, positive, and optimistic. Other antonyms for "am down the mouth" could include "content," "satisfied," or "pleased," which all suggest a person who is feeling content and fulfilled with their life. Expressions like "on top of the world" or "feeling great" also convey this sense of positivity and happiness. By using these antonyms, we can create a more nuanced and descriptive picture of a person's emotional state, and better understand their inner feelings and motivations.