obstinate; stubborn; inflexible
祖父は昔気質で一徹な人だった。
My grandfather was old-fashioned and stubborn.
一徹な性格が時に周囲と衝突を生む。
A stubborn personality sometimes causes clashes with others.
職人として一徹に技を磨き続けた。
He kept honing his craft with single-minded dedication.