stiffen in WordNet English dictionary

1. restrict
   "Tighten the rules"; "stiffen the regulations"
   stiffen  tighten  tighten up  constrain 
   
2. become stiff or stiffer
   "He stiffened when he saw his boss enter the room"
   stiffen 
   
3. make stiff or stiffer
   "Stiffen the cream by adding gelatine"
   stiffen