The Latins call me “porcus.” A sly reproof to anyone boasting, showing off, or trying to make himself appear greater than he is. The fable says that a wolf was going to devour a pig, when the pig observed that it was Friday, and no good Catholic would eat meat on a Friday. Going on together, the wolf said to the pig, “They seem to call you by many names.” “Yes,” said the pig, “I am called swine, grunter, hog, and I know not what besides. The Latins call me porcus. ” “Porpus, do they?” said the wolf, making an intentional blunder. “Well, porpoise is a fish, and we may eat fish on a Friday.” So saying, he devoured him without another word.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894