1) Attenuate 2) Damaged 3) Decreased 4) Dilute 5) Diluted 6) Diminished 7) Enervated 8) Impaired 9) Lessened 10) Reduced 11) Sapped 12) Thinned 13) Vitiated 14) Waned 15) Weak 16) Wore thin 1) Diminished 2) Enfeebled 3) Feeble 4) Helpless 5) Impaired 6) Impaired by diminution 7) Ineffectual 8) Lessened 9) Lost vigor 10) Made weak or weaker 11) Mixed with water 12) Powerless 13) Reduced in strength 14) Vitiated 15) Watered down 
• (imp. & p. p.) of Weaken
A horse which started off well in a race and was in a position from which it could win, but could not keep up that pace or keep up with the pace of the other runners and dropped back in the field. That horse is said to have weakened.
[adj] - made weak or weaker
adjective made weak or weaker