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):

enfeebling
    adj 1: causing debilitation [syn: debilitative, enervating,
           enfeebling, weakening]