Most ascertained means the information that is most accurate or reliable. The antonyms for this term can vary according to the context in which it is being used. If it is about a guess or a presumption, then the antonyms may be unverified, uncertain or dubious. However, if it is related to knowledge, then the antonyms can be vague, unclear or hazy. In the case of facts or figures, unproven, untested or baseless can be antonyms of most ascertained. The antonyms for most ascertained primarily depend on the level of certainty, accuracy, and truthfulness of the information being conveyed.