The Collaborative International Dictionary of English v.0.48:

Stiffener \Stiff"en*er\, n.
   One who, or that which, stiffens anything, as a piece of
   stiff cloth in a cravat.
   [1913 Webster]

WordNet (r) 3.0 (2006):

stiffener
    n 1: material used for stiffening something