前所未闻的；从未听说的；骇人听闻的

Unheard \Un*heard"\, a.
1. Not heard; not perceived by the ear; as, words unheard by
those present.

2. Not granted an audience or a hearing; not allowed to
speak; not having made a defense, or stated one's side of
a question; disregarded; unheeded; as, to condem? a man
unheard.

What pangs I feel, unpitied and unheard! --Dryden.

3. Not known to fame; not illustrious or celebrated; obscure.

Nor was his name unheard or unadored. --Milton.

{Unheard of}.
(a) Not heard of; of which there are no tidings.
(b) Unknown to fame; obscure. --Glanvill.