There are several synonyms for "unmentionably," which means something that is too unpleasant or taboo to speak of. One alternative word is "unspeakably," which conveys a similar sense of horror or disgust. Another option is "inappropriately," which suggests a breach of social norms or customs. Additionally, "taboo" and "forbidden" are other words that can be used to describe topics that are unmentionable. "Hushed," "shrouded," and "veiled" are also potential synonyms, as they convey a sense of secrecy or hiddenness. Ultimately, the choice of synonym will depend on the context and tone of the writing in which it is used.