expression, i-adjective
thickheaded; stubborn; inflexible
Describes a person who is rigid in thinking, unwilling to change their mind or consider new ideas. Often used critically.
See also: 頭の固い
彼は頭が固いから、新しいやり方をなかなか受け入れない。
He's so stubborn that he won't easily accept new ways of doing things.
そんなに頭が固いと、時代に取り残されるよ。
If you're that inflexible, you'll be left behind by the times.