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
   Metal.]
   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.
   [1913 Webster]

         A certain critical hour which shall . . . try what
         mettle his heart is made of.             --South.
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         Gentlemen of brave mettle.               --Shak.
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         The winged courser, like a generous horse,
         Shows most true mettle when you check his course.
                                                  --Pope.
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   To put one one's mettle, to cause or incite one to use
      one's best efforts.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mettle
    n 1: the courage to carry on; "he kept fighting on pure spunk";
         "you haven't got the heart for baseball" [syn: heart,
         mettle, nerve, spunk]