UNMENTIONABLE
Meaning of UNMENTIONABLE in English
un ‧ men ‧ tion ‧ a ‧ ble /ʌnˈmenʃ ə nəb ə l/ BrE AmE adjective
too shocking or embarrassing to talk about:
unmentionable scandals
Longman Dictionary of Contemporary English.
Longman - Словарь современного английского языка.
2012