1
general::
Very unusual, extraordinary, as in It's unheard of to have all one's money refunded two years after the purchase. This expression alludes to a circumstance so unusual that it has never been heard of. [Late 1500s]
American Heritage Idioms
2
general::
adj. VERBS be ADV. absolutely, completely, quite, totally At that time, it was completely unheard-of for girls to go to university. | by no means | almost, practically, virtually | hitherto, previously He was sentenced to a hitherto unheard-of seven life terms in prison.
Oxford Collocations Dictionary