noun, noun which may take the genitive case particle 'no'
Emphasizes that something is so extraordinary it has never been heard of before. Often used in formal or literary contexts.
これは希代未聞の大事件だ。
This is an unprecedented major incident.
彼の成功は希代未聞の快挙と言える。
His success can be called an unparalleled achievement.
Compound of 希代 (きたい, 'rare, unprecedented') and 未聞 (みもん, 'unheard-of'). The exact historical derivation is uncertain, but the combination reinforces the meaning of something never before seen or heard.