1892 (n.) "adherent of populism;" 1893 (adj.), American English, from Latin populus "people" (see people (n.)) + -ist. Originally in reference to the U.S. Populist Party organized February 1892 to promote certain issues important to farmers and workers (expansion of the currency, state control of railways, and restriction on the ownership of land). The term outlasted the party, and by 1920s came to mean "representing the views of the masses" in a general way.

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Definitions of populist from WordNet

populist (n.)
someone who advocates the rights of the common people over those of the elite;