you'll never guess

Interjection

idiomatic

(idiomatic) I have something very surprising to say; you won't be able to guess what I'm about to say.

Japanese Meaning

これから言う内容が非常に驚くべきもので、予測がつかないことを予告する表現。 / 今から述べることは信じがたく、誰も予想できないという期待と驚きを相手に抱かせる発言の前置き。

At lunch, you'll never guess what happened: my sister proposed to her boyfriend.

お昼に、信じられないかもしれないけど、姉が彼氏にプロポーズしたんだ。

Sense（1）

(idiomatic) I have something very surprising to say; you won't be able to guess what I'm about to say.