rack作为动词的时候有遭受剧烈痛苦；折磨的含义，因此形容让脑袋受到极端压力也就是努力想出什么东西，绞尽脑汁想出答案。

【例句】
She racked her brains, trying to remember exactly what she had said.
她绞尽脑汁，想要回忆起她到底说过些什么话。

Don't think this job a piece of cake. You'll have to rack your brains once you start on it.
别以为这工作是易如反掌的事，你做了就知道它是很伤脑筋的。