Wellfounded refers to something that has a strong and logical basis. Antonyms for the word wellfounded could include terms such as baseless, unfounded, groundless, and unsubstantiated. These words imply that the statement or belief lacks evidence or adequate support. By contrast, the word wellfounded implies that there is a solid rationale for the assertion, and it is built upon substantial facts or logical reasoning. Other synonyms for wellfounded might include valid, justified, reasonable, and sound. Using antonyms like unfounded or baseless suggests that the claim is without merit or credibility, highlighting the importance of evidence-backed reasoning.