Very stupid, or very slow to understand — with the extra idea that the person's head is solid, so nothing gets in. It is an insult, and quite a blunt one. It is not the cold clinical "low intelligence"; it is the exasperated feeling of explaining something for the third time and watching it bounce off.

Very stupid, or very slow to understand非常笨，或反應非常慢——with the extra idea that the person's head is solid還帶著這個人腦袋很硬的意思，so nothing gets in所以什麼都聽不進去。It它 is an insult是一種侮辱，and而且 quite a blunt one還滿直接、滿難聽的。It它 is not the cold clinical "low intelligence"不是那種冷冰冰、臨床式的「智力低」，it是 is the exasperated feeling of explaining something for the third time而是那種同一件事講到第三次、看著它完全聽不進去的不耐煩感覺。

Because of that, it is used most often about a particular moment or a particular refusal to see, rather than as a permanent judgement: How can you be so thick-headed? Don't be thick-headed — just read the instructions.

Because of that因為這樣，it is used most often about a particular moment or a particular refusal to see它最常是在講某個當下、某次不肯理解的狀況，rather than as a permanent judgement而不是永久性的評價：How can you be so thick-headed?你怎麼可以這麼thick-headed（這麼死腦筋）？Don't be thick-headed別thick-headed了——just read the instructions看說明書就好了。

Register: informal, spoken, and rude enough that you should not say it to a stranger, a colleague, or anyone senior to you. Between close friends it can be teasing, and people also say it about themselves, which is safe and rather charming: Sorry, I'm being thick-headed today — say that again?

Register語氣：informal, spoken口語、非正式，and而且 rude enough that you should not say it to a stranger, a colleague, or anyone senior to you不禮貌到不該對陌生人、同事、或長輩說。Between close friends在好朋友之間 it can be teasing可以是開玩笑，and而且 people also say it about themselves大家也會拿來說自己，which is safe and rather charming這樣講很安全、也滿可愛的：Sorry, I'm being thick-headed today — say that again?抱歉，我今天有點thick-headed（腦筋不靈光）——可以再說一次嗎？

There is a second, much rarer use meaning your head feels heavy and unclear, from a cold or from drinking too much — I woke up thick-headed — but the stupid meaning is the one you will actually meet. British English often just says thick with the same force.

There is a second, much rarer use還有第二種、少見很多的用法 meaning your head feels heavy and unclear指頭腦昏沉不清楚，from a cold or from drinking too much可能是感冒或喝太多酒造成的——I woke up thick-headed我起床時覺得頭昏昏的——but but the stupid meaning is the one you will actually meet不過你實際上會遇到的還是「笨」這個意思。British English英式英文 often just says thick with the same force常常直接說thick，力道一樣重。