The word "most defeasible" means something that is susceptible to being defeated or overridden. Antonyms for this word could be phrases such as "most impregnable," "most invulnerable," or "most unassailable." These words describe something that is unlikely to be overcome or undermined, often implying a sense of strength, stability, or security. Alternatively, antonyms for "most defeasible" could be phrases such as "most reliable," "most dependable," or "most trustworthy," which imply a sense of certainty or confidence in an outcome. Overall, the antonyms for "most defeasible" describe things that are secure, trustworthy, and unlikely to be challenged or undermined.