미상불 + statement (literary) ≈ 아닌 게 아니라 (spoken)
indeed, sure enough, truly
Example
미상불 그의 말이 옳았음이 밝혀졌습니다.
Sure enough, it turned out that he was right.
Word parts
미 not yet+상 ever+불 not
Literally 'never not' — so 'indeed, as expected'.
Good to know
Rare and bookish; in conversation say 아닌 게 아니라 or 과연.
Related words