E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Let Gryll be Gryll, and keep his hoggish mind. Dont attempt to wash a blackamoor white; the leopard will never change his spots. Gryll is from the Greek gru (the grunting of a hog). When Sir Guyon disenchanted the forms in the Bower of Bliss some were exceedingly angry, and one in particular, named Gryll, who had been metamorphosed by Acrasia into a hog, abused him most roundly. Come, says the palmer to Sir Guyon,
Let Gryll be Gryll, and have his hoggish mind.
But let us hence depart while weather serves, and wind.