The antonyms for the phrase "showing once for all" are numerous, but some of the most fitting include unclear, ambiguous, vague, indefinite, uncertain, and hesitant. When trying to convey a message or prove a point, using words and actions that are unclear or uncertain can leave others questioning your intentions or credibility. It is important to use language and behavior that is direct, precise, and conclusive to ensure that your message is clearly understood and not open to misinterpretation. By avoiding words and actions that are vague or uncertain, individuals can effectively communicate their thoughts and ideas and establish themselves as trustworthy and credible.