Dictionary.net

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.

Hence, an impostor in argument; a captious or fallacious reasoner.

One whose reasoning is clever but unsound.

A captious or fallacious reasoner.

One who argues cleverly but fallaciously.

n. [Greek] A philosopher; a teacher of logic, rhetoric, and philosophy ;-afterwards in ancient Greece one of an inferior class of men who taught for hire rhetoric and music; hence, as applied by Aristotle, a false teacher of philosophy ; a dealer in verbal niceties, quibbles, subtle enigmas, and fallacies; a captious or fallacious reasoner.