The phrase "on up-and-up" means to be honest and straightforward, but antonyms for this phrase include words such as deceitful, dishonest, and fraudulent. Other possible antonyms for the phrase include words such as sneaky, untrustworthy, and dubious. These words are often used to describe actions or behaviors that are intended to deceive or mislead others. By contrast, being on the up-and-up implies a level of transparency and integrity that is appreciated in both personal and professional settings. It is important to be aware of the antonyms for this phrase, as they can help individuals identify when someone is being deceitful or dishonest.