1. Lessen the strength of
   "The fever weakened his body"
    
2. Become weaker
   "The prisoner's resistance weakened after seven days"
    
3. Lessen in force or effect
   "weaken a shock";
   - dampen, damp, soften, break
    
4. Act to reduce, damage or destroy the effectiveness, power or stability of something
   "The Resistance weakened railway operations during the war";
   - sabotage, undermine, countermine, counteract, subvert

Derived forms: weakened, weakens, weakening

Type of: alter, blunt, change, come down, deaden, decrease, dent, diminish, disobey, fall, go down, lessen, modify, sink

Antonym: fortify, strengthen