Unmentionable is a word used to refer to things that are not appropriate or not accepted in a given setting. Some synonyms for this word include taboo, prohibited, forbidden, unspeakable, and beyond the pale. These words describe things that are not talked about openly due to their sensitive or controversial nature. Other synonyms for unmentionable include embarrassing, shameful, and inappropriate. The use of these words helps to convey the idea of something that is not acceptable in specific contexts. That said, it's important to note that the use of such words may vary depending on the culture and societal norms of a given group.