Yours, Etc.: Origins and Uses of 8 Sign-Offs
1510s, earlier plenar (mid-13c.), from Old French plenier, from Medieval Latin plenarius "entire, complete," from Latin plenus "full, filled, greatly crowded; stout, pregnant; abundant, abounding; complete," from PIE *pele- (1) "to fill" (see poly-). Related: Plenarily.