The term "down mouth" is a slang term that refers to a person who is pessimistic or negative in their outlook or expression. Antonyms for this term would include "optimistic," "positive," and "upbeat." An optimistic person maintains a positive outlook, which often leads to positive outcomes in life. Similarly, a positive person exudes positivity and hopefulness, which can inspire others around them. An upbeat person is energetic, cheerful, and enthusiastic, which can uplift those around them. Essentially, the opposite of a "down mouth" is someone who radiates positive vibes and encourages positivity in others.