What does "mettle" mean?

Mettle: Substance or quality of temperament; spirit, esp. as regards honor, courage, fortitude, ardor, etc.; disposition; -- usually in a good sense. A certain critical hour which shall... try what mettle his heart is made of. South. Gentlemen of brave mettle. Shak. The winged courser, like a generous horse, Shows most true mettle when you check his course. Pope. To put one one's mettle, to cause or incite one to use one's best efforts.