The Collaborative International Dictionary of English v.0.48:

Toughness \Tough"ness\, n.
   The quality or state of being tough.
   [1913 Webster]

WordNet (r) 3.0 (2006):

toughness
    n 1: enduring strength and energy [syn: stamina, staying
         power, toughness]
    2: the property of being big and strong [syn: huskiness,
       ruggedness, toughness]
    3: the elasticity and hardness of a metal object; its ability to
       absorb considerable energy before cracking [syn: temper,
       toughness]
    4: impressive difficulty [syn: formidability, toughness]

Moby Thesaurus II by Grady Ward, 1.0:

195 Moby Thesaurus words for "toughness":
   Spartanism, abstruseness, adamantness, adhesiveness, arduousness,
   astringency, austerity, authoritarianism, backbone, body,
   bothersomeness, bottom, bullheadedness, burdensomeness, callosity,
   callousness, chutzpah, clabbering, clamminess, clotting,
   coagulation, cohesiveness, colloidality, complexity, complication,
   concreteness, consistency, courage, curdling, decisiveness,
   demandingness, density, difficultness, difficulty, discipline,
   doughiness, dourness, durability, durity, endurance, energy,
   exactingness, firmness, flintiness, force, fortitude, gameness,
   gelatinity, gelatinousness, gluelikeness, gluiness, glutinosity,
   glutinousness, grimness, grit, gumlikeness, gumminess, guts,
   gutsiness, guttiness, hairiness, hard-bittenness, hard-nosedness,
   hardiness, hardness, hardness of heart, harshness, heart,
   heart of oak, heartiness, heaviness, immovability, immutability,
   impenetrability, implacability, impliability, incrassation,
   induration, inelasticity, inexorability, inflexibility,
   inspissation, intestinal fortitude, intransigeance, intransigence,
   intransigency, intransigentism, intricacy, irreconcilability,
   jellification, jellylikeness, laboriousness, lentor, lustihood,
   lustiness, mass, materiality, meticulousness, mettle,
   mettlesomeness, might, mightiness, moxie, mucilaginousness, nerve,
   obduracy, obstinacy, onerousness, oppressiveness, palpability,
   pastiness, persistence, persistency, pith, pluck, pluckiness,
   ponderability, potency, power, powerfulness, regimentation,
   relentlessness, resistance, restiveness, retention,
   rigid discipline, rigidity, rigor, rigorism, rigorousness,
   robustness, ropiness, ruggedness, severity, slabbiness, sliminess,
   snugness, solidity, soundness, spirit, spissitude, spunk,
   spunkiness, stability, stalwartness, stamina, staying power,
   steadiness, steeliness, sternness, stick-to-itiveness, stickiness,
   sticking power, stiff temper, stiffness, stodginess, stoniness,
   stout heart, stoutness, strength, strength of will, strenuousness,
   strictness, stringency, stringiness, stubbornness, sturdiness,
   substance, substantiality, substantialness, syrupiness, tackiness,
   tangibility, tenaciousness, tenacity, thickening, thickness,
   tightness, toilsomeness, treacliness, troublesomeness, true grit,
   unalterability, unbendingness, unchangeability, uncompromisingness,
   ungivingness, unrelentingness, unyieldingness, vigor, vigorousness,
   viscidity, viscosity, viscousness, vitality