When a certain action is deemed illegal or declared illegitimate, it can take its toll on the ones involved and the state of a particular situation. Some synonyms for declaring something as illegitimate include disavowing, renouncing, repudiating, nullifying, invalidating, and revoking. These terms ultimately imply that the action or decision has lost its credibility or authority, and it will no longer hold any weight or significance. It is important to use the appropriate synonym to accurately convey the intention of declaring something illegitimate to avoid any legal or personal troubles in the future.