The phrase "unmentionable thing" is often used to refer to something that is considered inappropriate, taboo, or shameful to discuss openly. However, there are many synonyms for this term that can be used to describe such topics or objects. Some other commonly used terms include "taboo subject," "forbidden topic," "unacceptable matter," "unspeakable topic," "inappropriate subject," "sensitive topic," and "controversial issue." These words help to convey the same message without directly using the specific term, making it easier to approach the topic without causing offense or discomfort. Knowing these synonyms can help you navigate challenging conversations or situations with greater ease and sensitivity.