Source : Webster's Revised Unabridged Dictionary (1913)

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.

            Stiffen the sinews, summon up the blood. --Shak.

   2. To inspissate; to make more thick or viscous; as, to
      stiffen paste.

   3. To make torpid; to benumb.

Stiffening \Stiff"en*ing\, n.
   1. Act or process of making stiff.

   2. Something used to make anything stiff.

   {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.

Source : WordNet®

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