unˈheard of BrE AmE , unheard-of adjective
something that is unheard of is so unusual that it has not happened or been known before:
Travel for pleasure was almost unheard of until the 19th century.
UNHEARD OF
Meaning of UNHEARD OF in English
Longman Dictionary of Contemporary English. Longman - Словарь современного английского языка. 2012