The Collaborative International Dictionary of English v.0.48:

deprecate \dep"re*cate\ (d[e^]p"r[-e]*k[=a]t), v. t. [imp. & p.
   p. Deprecated (-k[=a]`t[e^]d); p. pr. & vb. n.
   Deprecating (-k[=a]`t[i^]ng).] [L. deprecatus, p. p. of
   deprecari to avert by prayer, to deprecate; de- + precari to
   pray. See Pray.]
   To pray against, as an evil; to seek to avert by prayer; to
   seek deliverance from; to express deep regret for; to desire
   the removal of. [archaic]
   [1913 Webster]

   2. To protest against; to advance reasons against.
      [PJC]

            His purpose was deprecated by all round him, and he
            was with difficulty induced to adandon it. --Sir W.
                                                  Scott.
      [1913 Webster]

   3. To disapprove of strongly; to express a low opinion of.
      [PJC]

WordNet (r) 3.0 (2006):

deprecate
    v 1: express strong disapproval of; deplore
    2: belittle; "The teacher should not deprecate his student's
       efforts" [syn: deprecate, depreciate, vilipend]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "deprecate":
   belittle, bemoan, bewail, de-emphasize, denigrate, deplore,
   depreciate, derogate, despise, detract, discommend, discountenance,
   disdain, disesteem, disfavor, disparage, dispraise, disvalue,
   downplay, frown, hold in contempt, lament, make light of,
   make little of, make nothing of, minimize, misestimate, misprize,
   object, play down, put down, ridicule, sell short, set at naught,
   set little by, shrug off, think little of, think nothing of,
   underestimate, underplay, underprize, underrate, underreckon,
   undervalue