Nevertheless, one must also bear in mind the fact that, since Douglas was overworked, Olivia had a tedious, boring life and was about to crack up, when the Nawab came to rescue her.

crack 本义是"破裂、裂开"，在这里用作比喻意义，指"精神崩溃"。 up 在这里是副词，与动词 crack 搭配使用，crack up 是一个固定短语，意思是"精神崩溃、情绪失控"。 crack up 这个短语形象地描述了精神压力达到极限时突然崩溃的状态，就像物体承受过大压力时突然裂开一样。