This is the 10956th most frequent Korean word.
금시초문이다
"It’s the first time hearing about it."
The phrase is applied to indicate unfamiliarity with someone's reputation.
그가 유명한 작가라고 말하는데, 나는 금시초문이다.
They say he is a famous author, but that is news to me.
금시초문이다 is used here to express unfamiliarity with a story that was just heard.
그 이야기는 정말 금시초문이다.
That story is truly something I am hearing for the first time.
금시초문이다 is employed to highlight a surprising fact just learned.