Mulish (3-6)
(Other definitions for pig-headed that I've seen before include "Extremely obstinate" , "Obstinate in porcine way" .)
'mulish' is the definition.
(similar in meaning)
This is all the clue.
(Other definitions for obstinate that I've seen before include "such as a mule" , "headstrong" , "Female bird" , "Mulish" , "Cussed" .)