The phrase "down in the mouth" is often used to describe someone who is feeling sad, dejected, or discouraged. Some antonyms for this phrase could include "upbeat," "cheerful," or "optimistic." Someone who is upbeat may be described as being in high spirits or having a positive outlook on life. A cheerful person is generally happy and finds joy in life's small moments. An optimistic person has a tendency to see the bright side of situations and may be hopeful about the future. Ultimately, these antonyms serve to portray a person who is not down in the mouth and who may have a positive and hopeful demeanor.