WordWeb Online

1. Impaired by diminution
   - diminished, lessened, vitiated
    
2. Made weak or weaker
    
3. Reduced in strength
   "the weakened tones of an old recording"
   - attenuate, attenuated, faded
    
4. Mixed with water
   "sold weakened whiskey"
   - cut, thinned
    
5. Damaged inanimate objects or their value
   - hurt

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 or intensity or size or scope of
   "weaken a crisis"
   - de-escalate, step down
    
5. Lessen in force or effect
   "weaken a shock"
   - dampen, damp, soften, break

See also: damaged, decreased, dilute, diluted, impaired, reduced, weak, weakener, weakening

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

Antonym: strengthen