i-adjective
stubborn; inflexible; thickheaded
Describes a person who is rigid in thinking, unwilling to change their mind or accept new ideas. Often used critically. It is a sensitive expression.
See also: 頭が固い
He is a stubborn person and rarely accepts new ways of doing things.
そんな頭の固い考え方では、時代に取り残されるよ。
With such an inflexible way of thinking, you'll be left behind by the times.
Synonymous phrase using が instead of の. 頭の固い is the attributive form (modifying a noun), while 頭が固い is the predicative form (ending a sentence).
A set phrase combining 頭 (head) and 固い (hard, stiff), literally meaning 'hard-headed'. The figurative meaning of stubbornness is a natural extension.