strong determination; strong-willed; firmness of purpose
Describes a person or attitude marked by unwavering resolve and firm will. Often used in formal or written contexts.
He is a strong-willed person who sees through whatever he decides.
With firm determination, you can overcome any difficulty.
意志 is the general noun for 'will' or 'intention', while 意志堅固 specifically emphasizes unshakable firmness of will.
堅固 means 'solid' or 'firm' in a physical or abstract sense; 意志堅固 applies this firmness to one's will.
Yojijukugo composed of 意志 (will, intention) and 堅固 (firm, solid). The combination directly conveys an unyielding will.