WordWeb Online

1. Impaired by diminution
   - diminished, lessened, vitiated
    
2. Made weak or weaker
    
3. Reduced in strength
   - attenuate, attenuated, faded
    
4. Mixed with water
   - 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
   - sabotage, undermine, countermine, counteract, subvert
    
4. Reduce the level or intensity or size or scope of
   - de-escalate, step down
    
5. Lessen in force or effect
   - 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