verb (v)

• become stiff or stiffer(verb.change)

  Example:
  He stiffened when he saw his boss enter the room.
  

  Antonym:
  loose, loosen, relax
  

  source: wordnet30
• make stiff or stiffer(verb.change)

  Example:
  Stiffen the cream by adding gelatine.
  

  Antonym:
  loose, loosen
  

  source: wordnet30
• restrict(verb.change)

  Example:
  Stiffen the regulations.
  

  Synonym:
  constrain, tighten, tighten up
  

  source: wordnet30
• To make stiff; to make less pliant or flexible; as, to stiffen cloth with starch.(verb)

  source: webster1913
• To become stiff or stiffer, in any sense of the adjective.(verb)

  source: webster1913