The Collaborative International Dictionary of English v.0.48:

Stiffness \Stiff"ness\, n.
   The quality or state of being stiff; as, the stiffness of
   cloth or of paste; stiffness of manner; stiffness of
   character.
   [1913 Webster]

         The vices of old age have the stiffness of it too.
                                                  --South.
   [1913 Webster]

WordNet (r) 3.0 (2006):

stiffness
    n 1: the physical property of being inflexible and hard to bend
    2: the property of moving with pain or difficulty; "he awoke
       with a painful stiffness in his neck"
    3: firm resoluteness in purpose or opinion or action; "a
       charming host without any touch of stiffness or pomposity"
    4: the inelegance of someone stiff and unrelaxed (as by
       embarrassment) [syn: awkwardness, clumsiness,
       gracelessness, stiffness]
    5: excessive sternness; "severity of character"; "the harshness
       of his punishment was inhuman"; "the rigors of boot camp"
       [syn: severity, severeness, harshness, rigor,
       rigour, rigorousness, rigourousness, inclemency,
       hardness, stiffness]

Moby Thesaurus II by Grady Ward, 1.0:

140 Moby Thesaurus words for "stiffness":
   adamantness, aridity, barrenness, bloodlessness, buckram,
   ceremonial, ceremoniousness, ceremony, characterlessness,
   cohesiveness, colorlessness, costliness, deadness, dearness,
   dignity, dismalness, dourness, dragginess, dreariness, dryness,
   dullness, durability, dustiness, effeteness, emptiness, etiolation,
   extrinsicality, firmness, flatness, flintiness, form, formality,
   formalization, fundamentalism, gravity, great cost, grimness,
   hard-bittenness, hard-nosedness, hardiness, hardness, heaviness,
   highness, hollowness, immovability, immutability, impliability,
   inanity, inelasticity, inexcitability, inexorability,
   inflexibility, infrangibility, insipidity, insipidness,
   intransigeance, intransigence, intransigency, intransigentism,
   irreconcilability, jejunity, lastingness, leadenness,
   leatherlikeness, lifelessness, lowness of spirit, luxuriousness,
   obduracy, obdurateness, obstinacy, orthodoxy, paleness, pallor,
   pointlessness, pokiness, pomp, pomposity, ponderousness,
   precisianism, primness, purism, puritanism, relentlessness,
   renitence, renitency, resistance, richness, rigidity, rigidness,
   rigor, rigorism, rigorousness, ritual, rituality, ropiness,
   slowness, solemnity, spiritlessness, stamina, starchiness,
   steeliness, steepness, sterility, sternness, stiff temper,
   stiltedness, stodginess, strength, stringiness, stubbornness,
   stuffiness, stylization, sumptuousness, superficiality,
   tastelessness, tautness, tediousness, tenacity, tenseness, tension,
   tensity, tightness, toughness, unalterability, unbendingness,
   unbreakability, unbreakableness, unchangeability,
   uncompromisingness, ungivingness, uninterestingness, unliveliness,
   unrelentingness, unyieldingness, vapidity, vapidness, viscidity,
   vitality, weight, woodenness