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.
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   2. To inspissate; to make more thick or viscous; as, to
      stiffen paste.
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   3. To make torpid; to benumb.
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The Collaborative International Dictionary of English v.0.48:

Stiffen \Stiff"en\, v. i.
   To become stiff or stiffer, in any sense of the adjective.
   [1913 Webster]

         Like bristles rose my stiffening hair.   --Dryden.
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         The tender soil then stiffening by degrees. --Dryden.
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         Some souls we see,
         Grow hard and stiffen with adversity.    --Dryden.
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WordNet (r) 3.0 (2006):

stiffen
    v 1: become stiff or stiffer; "He stiffened when he saw his boss
         enter the room" [ant: loose, loosen, relax]
    2: make stiff or stiffer; "Stiffen the cream by adding gelatine"
       [ant: loose, loosen]
    3: restrict; "Tighten the rules"; "stiffen the regulations"
       [syn: stiffen, tighten, tighten up, constrain]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "stiffen":
   amplify, back, be tough, beef up, brace, brace up, buttress,
   case harden, charge, clot, coagulate, confirm, congeal,
   crystallize, electrify, endure, energize, fortify, galvanize,
   generate, gird, hang tough, harden, invigorate, jell, loop in,
   nerve, plug in, prop, refresh, reinforce, reinvigorate,
   restrengthen, rigidify, screw up, set, shock, shore up, short,
   short-circuit, solidify, steel, step down, step up, strengthen,
   support, sustain, switch off, switch on, tauten, temper, tense,
   thicken, tighten, toughen, trice up, turn off, turn on,
   undergird