rack的意思是“使受折磨，使痛苦不堪”，rack one's brains真正的意思就是“绞尽脑汁”，即苦思积虑，费尽脑筋，想尽办法，形容费尽心思去思考一件事情。

【例句】
We'll have to rack our brains and think of some effective measures.
我们得绞尽脑汁想出一些行之有效的办法。

He failed to find an answer after racking his brains long and hard over the question.
他苦思冥想，还是不得其解。