WordWeb Online

1. Lessen the strength of
   "The fever weakened his body"
    
2. Become weaker
   "The prisoner's resistance weakened after seven days"
    
3. Destroy property or hinder normal operations
   "The Resistance weakened railway operations during the war";
   - sabotage, undermine, countermine, counteract, subvert
    
4. Reduce the level, intensity, size or scope of
   "weaken a crisis";
   - de-escalate, step down
    
5. Lessen in force or effect
   "weaken a shock";
   - dampen, damp, soften, break

Derived forms: weakens, weakening, weakened

Type of: alter, blunt, change, deaden, decrease, diminish, disobey, fall, lessen, minify, modify

Antonym: fortify, intensify, strengthen