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. Restrict
   "stiffen the regulations"
   - tighten, tighten up, constrain

Derived forms: stiffens, stiffening, stiffened

See also: stiffener, stiffening

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

Antonym: loosen