Obdurate

/ˈɑːbdʊrət/ adjective hard

Stubbornly refusing to change one's mind or soften one's position.

Despite hours of pleading, the landlord remained obdurate about the rent increase.

Memory Trick

Latin 'durus' means hard (as in 'durable') — an obdurate person has a will as hard and unyielding as stone.