资料来源 : pyDict

无前例的,前所未闻的,空前的

资料来源 : Webster's Revised Unabridged Dictionary (1913)

Unheard-of \Un*heard"-of\, a.
   New; unprecedented; unparalleled. --Swift.

资料来源 : WordNet®

unheard-of
     adj : previously unknown; "a first novel by an unheard of writer";
           "developments on an unheard-of scale"