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.
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            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.
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         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]