Source : Webster's Revised Unabridged Dictionary (1913)

Anergia \An*er"gi*a\, Anergy \An"er*gy\, n. [NL. anergia, fr.
   Gr. ?- not + ? work.]
   Lack of energy; inactivity. -- {An*er"gic}, a.

Source : WordNet®

anergy
     n 1: reduction or lack of an immune response to a specific
          antigen
     2: inactivity and lack of energy