1640s, from Greek demagogos "popular leader," also "leader of the mob," from demos "people" (see demotic) + agogos "leader," from agein "to lead" (see act (n.)). Often a term of disparagement since the time of its first use, in Athens, 5c. B.C.E. Form perhaps influenced by French demagogue (mid-14c.).
by 1964, American English, from demagogue (n.). Related: Demagogued; demagoguing.