WordWeb Online

1. Become stiff or stiffer
   "He stiffened when he saw his boss enter the room"
    
2. Make stiff or stiffer
   "Stiffen the cream by adding gelatine"
    
3. (of laws or regulations) make more restrictive, or impose greater penalties
   "stiffen the regulations";
   - tighten, tighten up, constrain

Derived forms: stiffens, stiffening, stiffened

Type of: alter, bound, change, confine, limit, modify, restrict, throttle, trammel

Antonym: loose, relax