The antonyms for the word "more undoubtful" include doubtful, uncertain, indisputable, questionable, and dubious. If something is doubtful, it means there is a lack of trust or confidence in it. Uncertain means there is a lack of knowledge or information regarding it. Indisputable implies something is impossible to dispute or deny. Questionable suggests there's a doubt as to whether something is true or valid. Dubious implies something isn't quite believable or questionable in terms of reputability. Therefore, these antonyms can help to describe things that are uncertain, questionable, or improbable, and a synonym for more undoubtful could be more certain or more assured.