"of or characteristic of the lower class or the common people," 1560s in a Roman historical sense, from Latin plebeius "belonging to the plebs," earlier plebes, "the populace, the common people" (as opposed to patricians, etc.), also "commonality; the mass, the multitude; the lower class" (from PIE *ple-, from root *pele- (1) "to fill"). In general (non-historical) use from 1580s.
"member of the lowest class or the common people," 1530s, from Latin plebius "person not of noble rank," from adjective meaning "of the common people" (see plebeian (adj.)).