The word "nonpermissible" refers to something that is not allowed or is prohibited. There are several synonyms for this word that can be used interchangeably. Some of the synonyms include illegal, unauthorized, prohibited, banned, forbidden, and impermissible. These words all convey the same meaning as nonpermissible and can be used depending on the context in which they are needed. For instance, illegal and unauthorized can be used in situations where something is not allowed by law or regulation. On the other hand, prohibited, banned, and forbidden can be used in situations where something is not allowed due to ethical or moral reasons. Impermissible can be used when something is not allowed due to rules or regulations.