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]

WordNet (r) 3.0 (2006):

cynical
    adj 1: believing the worst of human nature and motives; having a
           sneering disbelief in e.g. selflessness of others [syn:
           cynical, misanthropic, misanthropical]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "cynical":
   Cassandra-like, Cassandran, Cassandrian, Rabelaisian, Timonistic,
   antisocial, captious, carping, caustic, caviling, critical,
   defeatist, dismal, downbeat, dry, faultfinding, gloomy,
   hairsplitting, hypercritical, ironic, man-hating, misogynous,
   nagging, negative, negativistic, niggling, nihilistic,
   overcritical, pessimist, pessimistic, pettifogging, quibbling,
   sarcastic, sardonic, satiric, sexist, trichoschistic,
   ultracritical, uncheerful, unsociable, woman-hating