The Collaborative International Dictionary of English v.0.48:

Spunk \Spunk\ (sp[u^][ng]k), n. [Gael. spong, or Ir. sponc,
   tinder, sponge; cf. AS. sponge a sponge (L. spongia), sp[=o]n
   a chip. Cf. Sponge, Punk.] [Written also sponk.]
   1. Wood that readily takes fire; touchwood; also, a kind of
      tinder made from a species of fungus; punk; amadou. --Sir
      T. Browne.
      [1913 Webster]

   2. An inflammable temper; spirit; mettle; pluck; as, a man of
      spunk. [Colloq.]
      [1913 Webster]

            A lawless and dangerous set, men of spunk, and
            spirit, and power, both of mind and body. --Prof.
                                                  Wilson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

spunk
    n 1: material for starting a fire [syn: kindling, tinder,
         touchwood, spunk, punk]
    2: 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:

86 Moby Thesaurus words for "spunk":
   German tinder, adventuresomeness, adventurousness, aggression,
   aggressiveness, amadou, ambitiousness, backbone, bang, bottle,
   bottom, chutzpah, courage, dash, dauntlessness, doggedness, drive,
   dynamism, enterprise, enterprisingness, fire, force, forcefulness,
   gameness, get-up-and-get, get-up-and-go, getup, ginger, go,
   go-ahead, go-getting, go-to-itiveness, grit, gumption, guts,
   gutsiness, guttiness, heart, heart of oak, hustle, initiative,
   intestinal fortitude, kick, marrow, mettle, mettlesomeness, moxie,
   nerve, pep, pepper, piss and vinegar, pith, pizzazz, pluck,
   pluckiness, poop, punch, punk, push, pushfulness, pushiness,
   pushingness, pyrotechnic sponge, resolution, resolve, sand, snap,
   spirit, spunkiness, stamina, starch, stout heart, thrust, tinder,
   tinder fungus, tinderbox, touchwood, toughness, true grit,
   up-and-comingness, venturesomeness, venturousness, verve, vim,
   zing, zip