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]