The Collaborative International Dictionary of English v.0.48:

Slumber \Slum"ber\, v. i. [imp. & p. p. Slumbered; p. pr. &
   vb. n. Slumbering.] [OE. slombren, slumberen, slumeren, AS.
   slumerian, fr. sluma slumber; akin to D. sluimeren to
   slumber, MHG. slummern, slumen, G. schlummern, Dan. slumre,
   Sw. slumra, Goth. slawan to be silent.]
   1. To sleep; especially, to sleep lightly; to doze. --Piers
      Plowman.
      [1913 Webster]

            He that keepeth Israel shall neither slumber nor
            sleep.                                --Ps. cxxi. 4.
      [1913 Webster]

   2. To be in a state of negligence, sloth, supineness, or
      inactivity. "Why slumbers Pope?" --Young.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "slumbering":
   abeyant, apathetic, asleep, cataleptic, catatonic, comatose, dead,
   dead asleep, deep asleep, dopey, dormant, dull, fast asleep,
   flaked-out, flat, foul, groggy, heavy, in abeyance, in suspense,
   inactive, inert, languid, languorous, latent, leaden, lifeless,
   logy, oblivious, out, passive, phlegmatic, sedentary, slack,
   sleeping, sluggish, smoldering, sound asleep, stagnant, standing,
   static, suspended, tame, torpid, unaroused, unconscious