Source : Webster's Revised Unabridged Dictionary (1913)

Toughness \Tough"ness\, n.
   The quality or state of being tough.

Source : WordNet®

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