The phrase "be up and up" typically denotes honesty, transparency, or sincerity. Since it represents a positive sentiment, its antonyms generally express deceit, dishonesty, or insincerity. A few possible opposites for "be up and up" could be "be shady," "be suspicious," "be deceitful," "be insincere," or "be dishonest." These antonyms suggest that someone is being intentionally misleading or untrue in their words or actions. In contrast, being "up and up" implies a person is candid, straightforward, and genuine, fostering trust and reliability in their dealings with others.