crack有“破裂、裂开”的意思，crack up可以表示“哈哈大笑；崩溃;（使）大笑;（因精神压力大而）垮掉”。

【例句】
She told some stories that cracked me up. 
她讲的故事让我哈哈大笑。

I kept a straight face as long as I could, but I finally had to crack up.
我尽量板着面孔，但最后还是控制不住失声大笑起来。