The term "unpermissible" refers to things that are not allowed or against the rules. If you're looking for synonyms to this word, you can use phrases like "prohibited," "forbidden," "disallowed," "banned," "unacceptable," or "off-limits." These words indicate that something is not permitted and that breaking the rules will lead to consequences. Another word that can be used is "inadmissible," which means that something is not acceptable as evidence in a legal case. All these synonyms of "unpermissible" suggest that there are boundaries or restrictions that one should follow to ensure compliance with rules or laws.