The Collaborative International Dictionary of English v.0.48:

Stiffening \Stiff"en*ing\, n.
   1. Act or process of making stiff.
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   2. Something used to make anything stiff.
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   Stiffening order (Com.), a permission granted by the
      customs department to take cargo or ballast on board
      before the old cargo is out, in order to steady the ship.
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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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WordNet (r) 3.0 (2006):

stiffening
    n 1: the act of becoming stiff; "stiffening his shoulders, he
         prepared to advance"
    2: the process of becoming stiff or rigid [syn: stiffening,
       rigidifying, rigidification]