The antonyms for the word "un-mentionable" would include words like acceptable, appropriate, approved, commendable, decent, and permissible. Something that is un-mentionable is generally considered taboo or inappropriate to discuss openly. The antonyms for this word suggest a positive connotation - something that is acceptable or approved. For example, if one is discussing social norms or appropriate behavior, un-mentionable topics might include things like profanity or indecency. On the other hand, if one is discussing ways to behave admirably or appropriately, antonyms for un-mentionable might include things like kindness, compassion, generosity, or consideration.