The Collaborative International Dictionary of English v.0.48:

Lassitude \Las"si*tude\ (l[a^]s"s[i^]*t[=u]d), n. [L. lassitudo,
   fr. lassus faint, weary; akin to E. late: cf. F. lassitude.
   See Late.]
   A condition of the body, or mind, when its voluntary
   functions are performed with difficulty, and only by a strong
   exertion of the will; languor; debility; weariness.
   [1913 Webster]

         The corporeal instruments of action being strained to a
         high pitch . . . will soon feel a lassitude. --Barrow.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lassitude
    n 1: a state of comatose torpor (as found in sleeping sickness)
         [syn: lethargy, lassitude, sluggishness]
    2: a feeling of lack of interest or energy [syn: languor,
       lassitude, listlessness]
    3: weakness characterized by a lack of vitality or energy [syn:
       inanition, lassitude, lethargy, slackness]