The Collaborative International Dictionary of English v.0.48:

Asthenic \As*then"ic\, a. [Gr. 'asqeniko`s; 'a priv. + sqe`nos
   strength.] (Med.)
   Characterized by, or pertaining to, debility; weak;
   debilitating.
   [1913 Webster]

WordNet (r) 3.0 (2006):

asthenic
    adj 1: lacking strength or vigor [syn: adynamic, asthenic,
           debilitated, enervated]
    2: having a slender physique