The Collaborative International Dictionary of English v.0.48:

Dissipate \Dis"si*pate\, v. i.
   1. To separate into parts and disappear; to waste away; to
      scatter; to disperse; to vanish; as, a fog or cloud
      gradually dissipates before the rays or heat of the sun;
      the heat of a body dissipates.
      [1913 Webster]

   2. To be extravagant, wasteful, or dissolute in the pursuit
      of pleasure; to engage in dissipation.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

dissipate \dis"si*pate\ (d[i^]s"s[i^]*p[=a]t), v. t. [imp. & p.
   p. Dissipated; p. pr. & vb. n. Dissipating.] [L.
   dissipatus, p. p. of dissipare; dis- + an obsolete verb
   sipare, supare. to throw.]
   1. To scatter completely; to disperse and cause to disappear;
      -- used esp. of the dispersion of things that can never
      again be collected or restored.
      [1913 Webster]

            Dissipated those foggy mists of error. --Selden.
      [1913 Webster]

            I soon dissipated his fears.          --Cook.
      [1913 Webster]

            The extreme tendency of civilization is to dissipate
            all intellectual energy.              --Hazlitt.
      [1913 Webster]

   2. To destroy by wasteful extravagance or lavish use; to
      squander.
      [1913 Webster]

            The vast wealth . . . was in three years dissipated.
                                                  --Bp. Burnet.

   Syn: To disperse; scatter; dispel; spend; squander; waste;
        consume; lavish.
        [1913 Webster]

WordNet (r) 3.0 (2006):

dissipate
    v 1: to cause to separate and go in different directions; "She
         waved her hand and scattered the crowds" [syn: disperse,
         dissipate, dispel, break up, scatter]
    2: move away from each other; "The crowds dispersed"; "The
       children scattered in all directions when the teacher
       approached"; [syn: disperse, dissipate, scatter,
       spread out]
    3: spend frivolously and unwisely; "Fritter away one's
       inheritance" [syn: fritter, frivol away, dissipate,
       shoot, fritter away, fool, fool away]
    4: live a life of pleasure, especially with respect to alcoholic
       consumption