The Collaborative International Dictionary of English v.0.48:

Populist \Pop"u*list\, n. [L. populus people + -ist.] (U. S.
   Politics)
   A member of the People's party. -- Pop`u*lis"tic, a.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

populist
    n 1: an advocate of democratic principles [syn: democrat,
         populist]

The Devil's Dictionary (1881-1906):

POPULIST, n.  A fossil patriot of the early agricultural period, found
in the old red soapstone underlying Kansas; characterized by an
uncommon spread of ear, which some naturalists contend gave him the
power of flight, though Professors Morse and Whitney, pursuing
independent lines of thought, have ingeniously pointed out that had he
possessed it he would have gone elsewhere.  In the picturesque speech
of his period, some fragments of which have come down to us, he was
known as "The Matter with Kansas."