The Collaborative International Dictionary of English v.0.48:

Dissembler \Dis*sem"bler\, n.
   One who dissembles; one who conceals his opinions or
   dispositions under a false appearance; a hypocrite.
   [1913 Webster]

         It is the weakest sort of politicians that are the
         greatest dissemblers.                    --Bacon.
   [1913 Webster]

         Priests, princes, women, no dissemblers here. --Pope.

   Syn: Dissembler, Hypocrite.

   Usage: A person is called a dissembler with reference to his
          concealment of his real character, and a hypocrite
          with reference to his assumption of a false character.
          But hypocrite is the stronger word, being commonly
          used to characterize a person who is habitually
          insincere and false, especially one who makes
          professions of goodness when his aims are selfish and
          his life corrupt.
          [1913 Webster]

WordNet (r) 3.0 (2006):

dissembler
    n 1: a person who professes beliefs and opinions that he or she
         does not hold in order to conceal his or her real feelings
         or motives [syn: hypocrite, dissembler, dissimulator,
         phony, phoney, pretender]