Of a person, or a person's heart, mind, or character: having the emotional or mental qualities necessary for dealing with difficult or distressing…

Of a person: patient, long-suffering, stoical; steadfast, resolute. Obsolete.

Of persons and their dispositions. Strong, capable of endurance. Obsolete.

In good sense: Strong-willed, strongly persistent. Obsolete. rare.

Having great intellectual or moral endurance; difficult to influence, affect, or impress; steadfast, firm, persistent; also, stubborn, obstinate…

Able to endure toil, fatigue, etc. Obsolete.

Having a strong, vigorous, or determined mind; not readily influenced by others, independent-minded.

Not giving way or departing from a position or principle; unyielding, inflexible, steady. Of persons.

figurative. Hard, insensible to fatigue; rigid, stern, unbending; ‘hard-and-fast’, unyielding, wanting in pliancy or adaptiveness. (Hyphenated.)

Having a backbone; vertebrate. Also figurative.