The quality or condition of being implacable.

The quality or condition of being steely.

The quality or character of being inexorable; incapability of being prevailed upon by entreaty; relentless or rigid severity.

Originally: strictness; stringency, severity; (later also of a system, etc.) the quality of being impervious to change; inflexibility, fixity. Also…

The quality, state, or condition of being rigid; strictness, severity; inflexibility, stiffness.

The quality of being inexorable; inexorability.

The quality of being implacable; implacability.

slang (originally and chiefly North American). Ruthless, uncompromising methods or dealings, esp. in politics. Frequently in to play hardball.