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]