The Collaborative International Dictionary of English v.0.48:

Cynic \Cyn"ic\ (s[i^]n"[i^]k), Cynical \Cyn"ic*al\ (-[i^]*kal),
   a. [L. cynicus of the sect of Cynics, fr. Gr. kyniko`s,
   prop., dog-like, fr. ky`wn, kyno`s, dog. See Hound.]
   1. Having the qualities of a surly dog; snarling; captious;
      currish.

            I hope it is no very cynical asperity not to confess
            obligations where no benefit has been received.
                                                  --Johnson.
      [1913 Webster]

   2. Pertaining to the Dog Star; as, the cynic, or Sothic,
      year; cynic cycle.
      [1913 Webster]

   3. Belonging to the sect of philosophers called cynics;
      having the qualities of a cynic; pertaining to, or
      resembling, the doctrines of the cynics.
      [1913 Webster]

   4. Given to sneering at rectitude and the conduct of life by
      moral principles; believing the worst of human nature and
      motives; disbelieving in the reality of any human purposes
      which are not suggested or directed by self-interest or
      self-indulgence; having a sneering disbelief in the
      selflessness of others; as, a cynical man who scoffs at
      pretensions of integrity; characterized by such opinions;
      as, cynical views of human nature.

   Syn: misanthropic, misanthropical.
        [1913 Webster]

   Note: In prose, cynical is used rather than cynic, in the
         senses 1 and 4.
         [1913 Webster]

   Cynic spasm (Med.), a convulsive contraction of the muscles
      of one side of the face, producing a sort of grin,
      suggesting certain movements in the upper lip of a dog.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Cynic \Cyn"ic\, n. (Gr. Philos)
   1. One of a sect or school of philosophers founded by
      Antisthenes, and of whom Diogenes was a disciple. The
      first Cynics were noted for austere lives and their scorn
      for social customs and current philosophical opinions.
      Hence the term Cynic symbolized, in the popular judgment,
      moroseness, and contempt for the views of others.
      [1913 Webster]

   2. One who holds views resembling those of the Cynics; a
      snarler; a misanthrope; particularly, a person who
      believes that human conduct is directed, either
      consciously or unconsciously, wholly by self-interest or
      self-indulgence, and that appearances to the contrary are
      superficial and untrustworthy.
      [1913 Webster]

            He could obtain from one morose cynic, whose opinion
            it was impossible to despise, scarcely any not
            acidulated with scorn.                --Macaulay.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cynic
    n 1: someone who is critical of the motives of others [syn:
         cynic, faultfinder]
    2: a member of a group of ancient Greek philosophers who
       advocated the doctrine that virtue is the only good and that
       the essence of virtue is self-control

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "cynic":
   Cassandra, Eeyore, Thersites, backbiter, belittler,
   calamity howler, calumniator, caustic critic, debunker, decrier,
   defamer, defeatist, deflater, depreciator, derogator, detractor,
   disparager, hatchet man, killjoy, knocker, libeler, malist,
   man-hater, misanthrope, misanthropist, misogynist, muckraker,
   mudslinger, negativist, nihilist, pessimist, prophet of doom,
   railer, retreatist, seek-sorrow, sexist, slanderer, social critic,
   traducer, woman-hater, worrywart



The Devil's Dictionary (1881-1906):

CYNIC, n.  A blackguard whose faulty vision sees things as they are,
not as they ought to be.  Hence the custom among the Scythians of
plucking out a cynic's eyes to improve his vision.