1. going beyond the requirements of duty.
2. greater than that required or needed; superfluous.
The manner of the Quartermaster had that air of supererogatory courtesy about it which almost invariably denotes artifice; for, while physiognomy and phrenology are but lame sciences at the best, the perhaps lead to as many false as right conclusions, we hold that there is no more infallible evidence of insincerity of purpose, short of overt acts, than a face that smiles when there is no occasion, and the tongue that is out of measure smooth. – James Fenimore Cooper, The Pathfinder, 1840
But you are always given to surprise me with abundant kindness–with supererogatory kindness. I believe in that, certainly. – Elizabeth Barrett Browning, Elizabeth Barrett Browning to H. S. Boyd, August 14, 1844
Supererogatory stems from the Medieval Latin superērogātōrius, with the root word ērogāre meaning “to pay out.” It entered English in the late 1500s.