The Collaborative International Dictionary of English v.0.48:
Mettle \Met"tle\, n. [E. metal, used in a tropical sense in
allusion to the temper of the metal of a sword blade. See
Substance or quality of temperament; spirit, esp. as regards
honor, courage, fortitude, ardor, etc.; disposition; --
usually in a good sense; as, to test a person's mettle.
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.
To put one one's mettle, to cause or incite one to use
one's best efforts.