WordWeb Online

1. Make strong or stronger
   "strengthen the relations between the two countries"; "This exercise will strengthen your upper body";
   - beef up, fortify
    
2. Gain strength
   "His body strengthened";
   - toughen
    
3. Give a healthy elasticity to
   "Let's strengthen our muscles";
   - tone, tone up

Derived forms: strengthens, strengthening, strengthened

Type of: alter, change, exercise, modify, work out

Antonym: weaken