Grand Mistress is a title in the Amazon culture.
Grand mistress obviously means the high honcho of either the pantheon church or the military branches of the Amazon culture.
It seems likely it's the title of high priestess of Athulua, or perhaps of each goddess or god, since the Grand Mistress Celestia demonstrated formidable abilities with her Fend to Initiates of Athulua by striking down a dozen experienced warriors in the space of an arrow's flight.