Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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