early 15c., plenarie, "full, complete," earlier plenar (mid-13c.), from Old French plenier and directly from Medieval Latin plenarius "entire, complete," from Latin plenus "full, filled, greatly crowded; stout, pregnant; abundant, abounding; complete," from PIE root *pele- (1) "to fill." Of an assembly, "fully attended," 1530s. Meaning "having full power" is from 1861. Related: Plenarily.