Synonyms for Toughness (same or very similar meaning)
WordNet sense 1 (enduring strength and energy):
stamina, staying power
WordNet sense 2 (the property of being big and strong):
huskiness, ruggedness
WordNet sense 3 (the elasticity and hardness of a metal object; its ability to absorb considerable energy before cracking):
temper
WordNet sense 4 (impressive difficulty):
formidability
From the ODE community, based on WordNet
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