Promising can be viewed as something that is likely to succeed or be successful, while un-promising is the opposite. Therefore, antonyms (words with opposite meanings) for un-promising are promising, hopeful, encouraging, auspicious, and propitious. These words all suggest a favorable outcome, which is the opposite of un-promising. For example, a startup company with a promising future has a great chance of success, while a company with an un-promising future is on the verge of failing. By using the right antonym, you can strengthen your writing and convey your message more clearly.