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.
   [1913 Webster]

         Gentlemen of brave mettle.               --Shak.
   [1913 Webster]

         The winged courser, like a generous horse,
         Shows most true mettle when you check his course.
                                                  --Pope.
   [1913 Webster]

   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]

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "mettle":
   activity, animation, animus, aptitude, ardor, backbone, bent, bias,
   bottom, brio, briskness, cast, character, chutzpah, constitution,
   courage, dauntlessness, diathesis, disposition, eccentricity, elan,
   enthusiasm, gameness, glow, grain, grit, gusto, guts, gutsiness,
   guttiness, heart, heart of oak, idiosyncrasy, impetuosity, impetus,
   inclination, individualism, intestinal fortitude, joie de vivre,
   kidney, leaning, life, liveliness, lustiness, make, makeup,
   mental set, mettlesomeness, mind, mind-set, mold, moxie, nature,
   nerve, perkiness, pertness, pith, pluck, pluckiness, predilection,
   predisposition, preference, proclivity, propensity, resolution,
   robustness, set, slant, spirit, spiritedness, spunk, spunkiness,
   stamina, stamp, stout heart, strain, streak, stripe, temper,
   temperament, tendency, toughness, true grit, turn, turn of mind,
   twist, type, vivacity, warmth, warp, zest, zestfulness