The word "un-dubitable" means something that is unquestionable or certain. There are many antonyms that express the opposite meaning of this word. Some of the most common antonyms of "un-dubitable" are doubtful, questionable, uncertain, ambiguous, indefinite, and dubious. A person who is uncertain about something may hesitate or question it, whereas someone who is convinced of something will not doubt it. So, if you want to express doubt or uncertainty about a particular matter, you can use these antonyms of "un-dubitable". These words will help you define your thoughts more efficiently and accurately.