The Collaborative International Dictionary of English v.0.48:

Lethargic \Le*thar"gic\ (l[-e]*th[aum]r"j[i^]k), Lethargical
\Le*thar"gic*al\ (-j[i^]*kal), a. [L. lethargicus, Gr.
   lhqargiko`s: cf. F. l['e]thargique. See Lethargy.]
   Pertaining to, affected with, or resembling, lethargy;
   morbidly drowsy; dull; heavy. -- Le*thar"gic*al*ly, adv. --
   Le*thar"gic*al*ness, n. -- Le*thar"gic*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lethargically
    adv 1: without energy; in a lethargic manner; "he hung around
           the house lethargically" [syn: lethargically,
           unenergetically]