The Collaborative International Dictionary of English v.0.48:

Enfeeble \En*fee"ble\, v. t. [imp. & p. p. Enfeebled; p. pr. &
   vb. n. Enfeebling.] [OF. enfeblir, enfeiblir; pref. en- (L.
   in) + feble, F. faible, feeble. See Feeble.]
   To make feeble; to deprive of strength; to reduce the
   strength or force of; to weaken; to debilitate.
   [1913 Webster]

         Enfeebled by scanty subsistence and excessive toil.
                                                  --Prescott.

   Syn: To weaken; debilitate; enervate.
        [1913 Webster]

WordNet (r) 3.0 (2006):

enfeeble
    v 1: make weak; "Life in the camp drained him" [syn: enfeeble,
         debilitate, drain]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "enfeeble":
   abate, afflict, attenuate, blunt, bugger, cramp, cripple, damp,
   dampen, de-energize, deaden, debilitate, derange, devitalize,
   disable, disenable, disorder, drain, dull, enervate, eviscerate,
   exhaust, extenuate, gruel, hamstring, hors de combat, hospitalize,
   inactivate, incapacitate, indispose, invalid, kibosh, lame,
   lay low, lay up, maim, mitigate, put, queer, queer the works,
   rattle, reduce, sabotage, sap, shake, shake up, sicken, soften up,
   spike, unbrace, undermine, unfit, unman, unnerve, unstrengthen,
   unstring, weaken, wing, wreck