The Collaborative International Dictionary of English v.0.48:

Stiffening \Stiff"en*ing\, n.
   1. Act or process of making stiff.
      [1913 Webster]

   2. Something used to make anything stiff.
      [1913 Webster]

   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.
      [1913 Webster]