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stubborn; obstinate; pigheaded
祖父はとても頑固な性格の人でした。
My grandfather had a stubborn personality.
父は頑固な性格で、一度決めたら意見を変えません。
My father is stubborn and won't change his opinion.
服に付いた頑固な汚れが落ちない。
The stubborn stain on my clothes won't come off.