The Collaborative International Dictionary of English v.0.48:

Hardiness \Har"di*ness\ (-d[i^]*n[e^]s), n.
   1. Capability of endurance.
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   2. Hardihood; boldness; firmness; assurance. --Spenser.
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            Plenty and peace breeds cowards; Hardness ever
            Of hardiness is mother.               --Shak.
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            They who were not yet grown to the hardiness of
            avowing the contempt of the king.     --Clarendon.
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   3. Hardship; fatigue. [Obs.] --Spenser.
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WordNet (r) 3.0 (2006):

hardiness
    n 1: the property of being strong and healthy in constitution
         [syn: robustness, hardiness, lustiness, validity]
    2: 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:

66 Moby Thesaurus words for "hardiness":
   assurance, audacity, brashness, bulldog courage, cohesiveness,
   decisiveness, durability, endurance, energy, force, fortitude,
   gameness, gaminess, grittiness, guts, gutsiness, haleness,
   hardihood, hardness, heartiness, infrangibility,
   intestinal fortitude, lastingness, leatherlikeness, longevity,
   lustihood, lustiness, mettlesomeness, might, mightiness, nerve,
   nerviness, obstinacy, pertinaciousness, pertinacity, pluckiness,
   potency, power, powerfulness, resistance, resoluteness, resolution,
   robustness, ropiness, ruggedness, spunkiness, stalwartness,
   stamina, staying power, sticking power, stiffness, stoutness,
   strength, strength of will, stringiness, stubbornness, sturdiness,
   tenaciousness, tenacity, toughness, unbreakability,
   unbreakableness, vigor, vigorousness, viscidity, vitality