The Collaborative International Dictionary of English v.0.48:

Exacerbate \Ex*ac"er*bate\ ([e^]gz*[a^]s"[~e]r*b[=a]t; 277), v.
   t. [imp. & p. p. Exacerbated
   ([e^]gz*[a^]s"[~e]r*b[=a]`t[e^]d); p. pr. & vb. n.
   Exacerbating ([e^]gz*[a^]s"[~e]r*b[=a]`t[i^]ng).] [L.
   exacerbatus, p. p. of exacerbare; ex out (intens.) +
   acerbare. See Acerbate.]
   To render more violent or bitter; to irritate; to exasperate;
   to imbitter, as passions or disease. --Brougham.
   [1913 Webster]

WordNet (r) 3.0 (2006):

exacerbate
    v 1: make worse; "This drug aggravates the pain" [syn: worsen,
         aggravate, exacerbate, exasperate] [ant:
         ameliorate, amend, better, improve, meliorate]
    2: exasperate or irritate [syn: exacerbate, exasperate,
       aggravate]