NOUN (3)

1. the state of being vigorous and robust;

2. the property of something that is strongly built;

3. resoluteness evidenced by strength of character;
- Example: "sturdiness of moral principle"

The Collaborative International Dictionary of English v.0.48:

Sturdiness \Stur"di*ness\, n.
   Quality of being sturdy.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sturdiness
    n 1: the state of being vigorous and robust
    2: the property of something that is strongly built
    3: resoluteness evidenced by strength of character; "sturdiness
       of moral principle"

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "sturdiness":
   body, concreteness, decisiveness, density, durability, endurance,
   energy, firmness, force, fortitude, guts, gutsiness, hardiness,
   hardness, heartiness, intestinal fortitude, lustihood, lustiness,
   mass, materiality, might, mightiness, obstinacy, palpability,
   ponderability, potency, power, powerfulness, robustness,
   ruggedness, solidity, soundness, stability, stalwartness, stamina,
   staunchness, staying power, steadiness, sticking power, stoutness,
   strength, strength of will, substance, substantiality,
   substantialness, tangibility, temper, toughness, vigor,
   vigorousness, vitality