The Collaborative International Dictionary of English v.0.48:

Stiffen \Stiff"en\, v. t. [imp. & p. p. Stiffened; p. pr. &
   vb. n. Stiffening.] [See Stiff.]
   1. To make stiff; to make less pliant or flexible; as, to
      stiffen cloth with starch.
      [1913 Webster]

            Stiffen the sinews, summon up the blood. --Shak.
      [1913 Webster]

   2. To inspissate; to make more thick or viscous; as, to
      stiffen paste.
      [1913 Webster]

   3. To make torpid; to benumb.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "stiffened":
   accelerated, aggrandized, amplified, augmented, backed, beefed-up,
   bloated, boosted, broadened, calcified, callous, calloused,
   case-hardened, crusted, crusty, crystallized, deepened, elevated,
   enhanced, enlarged, expanded, extended, fossilized, granulated,
   hardened, heightened, hiked, hornified, increased, incrusted,
   indurate, indurated, inflated, intensified, jazzed up, lapidified,
   magnified, multiplied, ossified, petrified, proliferated, raised,
   reinforced, rigidified, sclerotic, set, solidified, spread,
   steeled, strengthened, swollen, tightened, toughened, vitrified,
   widened