不说不知道一说吓一跳

不说不知道，一说吓一跳:

Literally means:

don't know until you hear it, the moment you know, you get a shock Actually means:

It means that something is surprising when it is let known. It figuratively refers to an unusual or very serious matter. It implies that at first people did not know it or did not understand how serious it was.

说出来的事让人很吃惊。比喻事情不一般或比较严重。常含有原先对事情并不知道，或不了解其严重性等意思。