The Collaborative International Dictionary of English v.0.48:

Enfeeblement \En*fee"ble*ment\, n.
   The act of weakening; enervation; weakness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

enfeeblement
    n 1: serious weakening and loss of energy [syn: debilitation,
         enervation, enfeeblement, exhaustion]