The Collaborative International Dictionary of English v.0.48:

Hardihood \Har"di*hood\ (h[aum]r"d[i^]*h[oo^]d), n. [Hardy +
   -hood.]
   Boldness, united with firmness and constancy of mind;
   bravery; intrepidity; also, audaciousness; impudence.
   [1913 Webster]

         A bound of graceful hardihood.           --Wordsworth.
   [1913 Webster]

         It is the society of numbers which gives hardihood to
         iniquity.                                --Buckminster.

   Syn: Intrepidity; courage; pluck; resolution; stoutness;
        audacity; effrontery; impudence.
        [1913 Webster]

WordNet (r) 3.0 (2006):

hardihood
    n 1: the trait of being willing to undertake things that involve
         risk or danger; "the proposal required great boldness";
         "the plan required great hardiness of heart" [syn:
         boldness, daring, hardiness, hardihood] [ant:
         timidity, timorousness]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "hardihood":
   assurance, audacity, birr, boldness, brashness, brazenness,
   cockiness, disrespect, fortitude, go, grit, guts, hardiness,
   impertinence, impudence, intrepidity, moxie, nerve, pep, pluck,
   potency, sand, tuck, vigor