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.

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

         Like bristles rose my stiffening hair.   --Dryden.

         The tender soil then stiffening by degrees. --Dryden.

         Some souls we see, Grow hard and stiffen with
         adversity.                               --Dryden.

Source : WordNet®

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