immalleable
/ɪˈmalɪəbl/adjective
Not malleable; (originally) literal incapable of being hammered out or beaten; (in later use also of a person, the mind, etc.) unchangeable, obstinate, unyielding.
Origin
Mid 17th century; earliest use found in John French (?1616–1657), physician. From im- + malleable, originally after post-classical Latin immalleabilis.