Definition of unheard-of adjective from the Oxford Advanced Learner's Dictionary

 

unheard-of

 adjective
adjective
BrE BrE//ʌnˈhɜːd ɒv//
 
; NAmE NAmE//ʌnˈhɜːrd ʌv//
 
 
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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. At that time, it was completely unheard-of for girls to go to university. He was sentenced to a hitherto unheard-of seven life terms in prison.
See the Oxford Advanced American Dictionary entry: unheard-of