The Collaborative International Dictionary of English v.0.48:

Hebetude \Heb"e*tude\, n. [L. hebetudo.]
   Dullness; stupidity. --Harvey.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hebetude
    n 1: mental lethargy or dullness

Moby Thesaurus II by Grady Ward, 1.0:

109 Moby Thesaurus words for "hebetude":
   acedia, aloofness, apathy, asininity, ataraxia, ataraxy,
   benumbedness, blah, blahs, blockishness, boobishness, boorishness,
   boredom, bovinity, cloddishness, coma, comatoseness, crassness,
   crudeness, denseness, density, detachment, dim-wittedness, dimness,
   disinterest, dispassion, doltishness, drowsiness, dull-wittedness,
   dullardism, dullness, dumbness, duncery, enervation, ennui,
   fatigue, grossness, heartlessness, heaviness, hopelessness,
   inanimation, inappetence, indifference, insouciance, jadedness,
   lack of appetite, lackadaisicalness, languidness, languishment,
   languor, languorousness, lassitude, lenitude, lentor,
   lethargicalness, lethargy, lifelessness, listlessness, loutishness,
   lumpishness, ninnyism, nonchalance, numbness, oafdom, oafishness,
   obtuseness, opacity, oscitancy, passiveness, passivity, phlegm,
   phlegmaticalness, phlegmaticness, plucklessness, resignation,
   resignedness, satedness, simpletonianism, sleep, sleepiness, sloth,
   slothfulness, slowness, sluggishness, slumber, somnolence, sopor,
   soporifousness, sottishness, spiritlessness, spunklessness,
   stolidity, stupefaction, stupidity, stupor, supineness,
   thick-headedness, thick-wittedness, torpidity, torpidness,
   torpitude, torpor, unconcern, unteachability, weariness,
   withdrawnness, world-weariness, wrongheadedness, yokelism