The word "de-feasible" is often used in legal contexts to describe situations where an obligation or contractual agreement can be terminated or canceled. Therefore, the antonyms for "de-feasible" would be words that describe situations or agreements that cannot be canceled or voided. Some antonyms could include words like binding, permanent, mandatory, non-negotiable, irrevocable, or void-proof. These words indicate that the obligation or agreement in question is final and cannot be changed or invalidated. These antonyms are essential in legal and contractual settings, as they help create clarity and stability in agreements between parties.