unmentionable
adjective uk/ʌnˈmen.ʃən.ə.bəl/ us/ʌnˈmen.ʃən.ə.bəl/
不宜提及的；难以启齿的
shocking or embarrassing, and therefore not allowed or disapproved of as a subject of conversation
the unmentionable possibility that we might lose the war 我们可能战败这一难以启齿的可能性