So shocking, rude, or embarrassing that people will not say it out loud. It is used of subjects rather than of single words — death in some families, money in others, an illness nobody names, a relative everyone has agreed to forget. What makes the word interesting is that it is almost never a plain report; the speaker is usually half-smiling at the silence. When an English speaker says 'we never discussed the unmentionable subject of Grandfather's first wife', the taboo is being described and gently mocked at the same time. Used with a completely straight face, it belongs to older or more formal writing. There is a real difference between 'unmentionable' and 'unspeakable': unspeakable means so terrible that words fail — unspeakable cruelty — while unmentionable means merely not allowed in polite conversation. One is horror, the other is manners. It also works as a noun for the thing itself, though that is rare in the singular; the plural has a life of its own and is listed separately.

難以啟齒的、不便說出口的（禁忌話題）