Defeasible is a term that is often used in legal language, referring to a law or contract that can be cancelled or invalidated in certain circumstances. Some antonyms for this term include indisputable, irrefutable, incontrovertible, and unassailable. These words suggest that something is absolute and cannot be disputed or challenged in any way. Other antonyms for defeasible might include words like certain, definite, or dependable, indicating that there is no room for doubt or revision. Overall, the antonyms of defeasible are words that suggest certainty, reliability, and immutability, diametrically opposed to the idea of something being subject to cancellation or invalidation.