Mettle \Met"tle\, n. [E. metal, used in a tropical sense in allusion to the temper of the metal of a sword blade. See Metal.]

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.

Source: Webster's Revised Unabridged Dictionary (1913)