/ ʌnˈhɜːd ɒv; NAmE ʌnˈhɜːrd ʌv/ adjective
that has never been known or done; very unusual :
He'd dyed his hair, which was almost unheard-of in the 1960s.
It is almost unheard-of for a new band to be offered such a deal.
UNHEARD-OF
Meaning of UNHEARD-OF in English
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005