We had a knocker up and our knocker up had a knocker up.

有个敲窗人，他自己也有个敲窗人。

And our knocker up's knocker up didn't knock our knocker up.

有一天敲窗人的敲窗人没有叫我们的敲窗人起床。

So that's where the knocker up, or the Knocker Upper came in.

于是“敲窗人”这个工种就应运而生了。