Don't be such a knuckle-head, you got such an easy question wrong.

knuckle 本意是"指关节"，head 是"头"的意思。 knuckle-head 是由 knuckle 和 head 组成的复合词，字面意思是"指关节脑袋"，用来形容一个人头脑简单、愚蠢，相当于中文里的"笨蛋"或"傻瓜"。