The Collaborative International Dictionary of English v.0.48:

Sophist \Soph"ist\, n. [F. sophiste, L. sophistes, fr. Gr. ?.
   See Sophism.]
   1. One of a class of men who taught eloquence, philosophy,
      and politics in ancient Greece; especially, one of those
      who, by their fallacious but plausible reasoning, puzzled
      inquirers after truth, weakened the faith of the people,
      and drew upon themselves general hatred and contempt.
      [1913 Webster]

            Many of the Sophists doubdtless card not for truth
            or morality, and merely professed to teach how to
            make the worse appear the better reason; but there
            scems no reason to hold that they were a special
            class, teaching special opinions; even Socrates and
            Plato were sometimes styled Sophists. --Liddell &
                                                  Scott.
      [1913 Webster]

   2. Hence, an impostor in argument; a captious or fallacious
      reasoner.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Sophist
    n 1: any of a group of Greek philosophers and teachers in the
         5th century BC who speculated on a wide range of subjects
    2: someone whose reasoning is subtle and often specious [syn:
       casuist, sophist]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "sophist":
   Jesuit, casuist, choplogic, cosmologist, dialectician, logicaster,
   logician, logistician, metaphysician, paralogist, philosophaster,
   philosophe, philosopher, philosophizer, ratiocinator, rationalist,
   rationalizer, reasoner, sophister, speculator, syllogist,
   syllogizer, thinker