It’s the Latin version of “you gotta hear this” or “if you can believe this” or anything along those lines—even “holy cow” in the right context.

mirabile dictu (mi·ra·bi·le dic·tu): / mi RAHB uh lee DIK too / (always italicized)
interjection

(1) expression of wonder: introduction to something the speaker feels is amazing (marvelous to relate)
(2) expression of disbelief: introduction to something the speaker feels is difficult to believe (strange to say)

[From Latin mirabile dictu meaning “amazing to relate” (literally, “amazing/marvelous in the saying”).]

Usage: And then, mirabile dictu, he jumped from the roof, fell three stories, landed on his feet, and promptly walked away as though nothing happened.