I think,
Rockhead is 돌머리
rock is 단단한 but sb usually accent "ㄷ" to "ㄸ" and read this "딴딴한".
but writing and read this " 단단한".
단단한 is correct but that I read this 딴딴한 is more convienience.
I think....
Sb more using dialect, the more read this "딴딴한".
but 단단한 is correct
and,,, rockhead translate 단단한 머리 = 돌머리
Rockhead's slang form is 돌대가리
대가리(= head) is slang
Becaus 대가리 point animal's head such as a cow's head
Some people who hear that their head are 대가리 are very angry.
I think that rockhead similar to stone head
do you know stone head?
I translate 돌머리 to stone-head in English
I think stone-head is Konglish;;;;;;kkkk
Rockhead man is very stupid and obstinate isn't it?
I hope helping to you......