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]

The Collaborative International Dictionary of English v.0.48:

enfeebled \enfeebled\ adj.
   same as debilitated, 2.

   Syn: debilitated, infirm, seedy.
        [WordNet 1.5]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "enfeebled":
   ausgespielt, burned-out, debilitated, devitalized, disabled,
   drained, drooping, droopy, effete, enervated, eviscerated,
   exhausted, fagged, faint, fainting, fatigued, feeling faint,
   flagging, footsore, frazzled, good and tired, incapacitated, jaded,
   languid, played out, ready to drop, run ragged, run-down, sagging,
   sapped, seedy, spent, tired, tired-winged, toilworn, unrefreshed,
   unrestored, used up, wasted, way-weary, wayworn, weak, weakened,
   wearied, weariful, weary, weary-footed, weary-laden, weary-winged,
   weary-worn, wilting, worn, worn-down, worn-out