摸不着头脑 at a loss
Explanation
形容对某事完全不明白，不知道是怎么回事。
Describes a situation where one doesn't understand something at all or doesn't know what's going on.
Origin Story
贾宝玉被父亲贾政严厉管教，强迫他读书。宝玉心中毫无兴趣，只是随意翻阅书籍，一会儿看《诗经》，一会儿看《楚辞》，一会儿又看话本小说，毫无章法。袭人在一旁侍候，看着宝玉毫无头绪地翻书，心中也是摸不着头脑，不知道该如何是好。她实在看不下去了，便轻声劝道：『宝玉，你这样看书，如何能读懂呢？不如放下书本，出去走走吧！』宝玉听了，这才放下书本，起身出去散步。袭人看着宝玉的背影，仍然是摸不着头脑，但她知道，宝玉需要的是自由，而不是强迫。
Jia Baoyu was strictly disciplined by his father, Jia Zheng, who forced him to study. Baoyu had no interest in it and just flipped through books randomly, sometimes reading the Classics, sometimes novels, without any method. Xiren waited on him and watched Baoyu flip through the books without any plan. She didn't understand the situation either and didn't know what to do. Finally, she couldn't stand it anymore and whispered: "Baoyu, you can't read like this. It's pointless. Put down the books and go for a walk!" Baoyu listened to her, put down the books, and went for a walk. Xiren watched Baoyu's back and still didn't understand the situation, but she knew that Baoyu needed freedom, not coercion.
Usage
常用来形容对事情完全搞不清楚，不明白是怎么回事。
Often used to describe that one doesn't understand something at all or doesn't know what's going on.
Examples
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他做事总是摸不着头脑，效率很低。
ta zuòshì zǒngshì mō bu zháo tóu nǎo, xiàolǜ hěn dī
He always does things without a clear idea, and his efficiency is very low.
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对于这个复杂的难题，我完全摸不着头脑。
duìyú zhège fùzá de nántí, wǒ wánquán mō bu zháo tóu nǎo
I'm completely at a loss with this complex problem.