E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A giant who lived with Hocus-Pocus in an enchanted castle. By his magic he changed men and women into dumb animals, amongst which was a dukes daughter, changed into a roe. Jack the Giant Killer, arrayed in his cap, which rendered him invisible, went to the castle and read the inscription: Whoever can this trumpet blow, will cause the giants overthrow. He seized the trumpet, blew a loud blast, the castle fell down, Jack slew the giant, and was married soon after to the dukes daughter, whom he had rescued from the giants castle. (Jack the Giant Killer.)