crack有破裂、裂开的意思。人的笑就像是严肃的表情裂开了，所以crack up 也有哈哈大笑的意思。

If you crack up or if someone or something cracks you up, you laugh a lot

She told stories that cracked me up and I swore to write them down so you could enjoy them too.

她讲了些让我哈哈大笑的故事，我发誓要把那些故事写下来，以便你们也可以欣赏。