Unreturnable, a word that refers to something that cannot be returned, can be replaced with several synonyms. These include nonreturnable, irreversible, irreparable, final, and non-reversible. Nonreturnable is a common synonym for unreturnable that conveys the idea that an item cannot be returned for any reason. Irreversible and irreparable both suggest a sense of finality and indicate that the situation cannot be undone or repaired. Final is also a similarly used option that is often used in the context of decisions or judgments. On the other hand, non-reversible also conveys a similar sense that something cannot be reversed or undone.