The antonym for the word "un doubtful" is "doubtful." If someone is doubtful, it means they are uncertain or hesitant about something. They may have reservations or skepticism about an idea, action, or statement. Doubtfulness can arise from lack of information, experience, or trust. It is a feeling of uncertainty that may prevent someone from committing to a decision. On the contrary, when someone is undoubtful, it means they are confident, certain, and convinced of what they believe or do. They have no reservations or hesitations, and they trust their judgment. Being undoubtful helps individuals make decisions promptly and execute them with confidence.