Tales of monsters and heroes are abundant around the world, making the source of "Jack the Giant Killer" difficult to pin down.
 However, the ascription of Jack's relation to Cornwall suggests a Brythonic (Celtic) origin.
The early Welsh tale How Culhwch won Olwen (tentatively dated to c. 1100), set in Arthurian Britain places Arthur as chief among the kings of Britain.
The young hero Culhwch ap Cilydd makes his way to his cousin Arthur's court at Celliwig in Cornwall where he demands Olwen as his bride; the beautiful daughter of the giant Ysbaddaden Ben Cawr ('Chief of Giants').
The Giant sets a series of impossible tasks which Arthur's champions Bedwyr and Cai are honour-bound to fulfill before Olwen is released to the lad; and the Giant King must die.
Folklorists Iona and Peter Opie have observed in The Classic Fairy Tales (1974) that "the tenor of Jack's tale, and some of the details of more than one of his tricks with which he outwits the giants, have similarities with Norse mythology."
An incident between Thor and the giant Skrymir in the Prose Edda of ca.
1220, they note, resembles the incident between Jack and the stomach-slashing Welsh giant.
The Opies further note that the Swedish tale of "The Herd-boy and the Giant" shows similarities to the same incident, and "shares an ancestor" with the Grimms's "The Valiant Little Tailor", a tale with wide distribution.
According to the Opies, Jack's magical accessories – the cap of knowledge, the cloak of invisibility, the magic sword, and the shoes of swiftness – could have been borrowed from the tale of Tom Thumb or from Norse mythology, however older analogues in British Celtic lore such as Y Mabinogi and the tales of Gwyn ap Nudd, cognate with the Irish Fionn mac Cumhaill, suggest that these represent attributes of the earlier Celtic gods such as the shoes associated with triple-headed Lugus; Welsh Lleu Llaw Gyffes of the Fourth Branch, Arthur's invincible sword Caledfwlch and his Mantle of Invisibility Gwenn one of the Thirteen Treasures of the Island of Britain mentioned in two of the branches; or the similar cloak of Caswallawn in the Second Branch.
Another parallel is the Greek demigod Perseus, who was given a magic sword, the winged sandals of Hermes and the 'cap of darkness' (borrowed from Hades) to slay the gorgon Medusa.
 Ruth B. Bottigheimer observes in The Oxford Companion to Fairy Tales that Jack's final adventure with Galigantus was influenced by the "magical devices" of French fairy tales.
The Opies conclude that analogues from around the world "offer no surety of Jack's antiquity."


The Opies note that tales of giants were long known in Britain.
King Arthur's encounter with the giant of St Michael's Mount  – or Mont Saint-Michel in Brittany – was related by Geoffrey of Monmouth in Historia Regum Britanniae in 1136, and published by Sir Thomas Malory in 1485 in the fifth chapter of the fifth book of Le Morte d'Arthur:
Then came to [King Arthur] an husbandman ... and told him how there was ... a great giant which had slain, murdered and devoured much people of the country ... [Arthur journeyed to the Mount, discovered the giant roasting dead children,] ... and hailed him, saying ... [A]rise and dress thee, thou glutton, for this day shalt thou die of my hand.
Then the glutton anon started up, and took a great club in his hand, and smote at the king that his coronal fell to the earth.
And the king hit him again that he carved his belly and cut off his genitours, that his guts and his entrails fell down to the ground.
Then the giant threw away his club, and caught the king in his arms that he crushed his ribs ... And then Arthur weltered and wrung, that he was other while under and another time above.
And so weltering and wallowing they rolled down the hill till they came to the sea mark, and ever as they so weltered Arthur smote him with his dagger.

Anthropophagic giants are mentioned in The Complaynt of Scotland in 1549, the Opies note, and, in King Lear of 1605, they indicate, Shakespeare alludes to the Fee-fi-fo-fum chant (" ... fie, foh, and fumme, / I
Tales of monsters and heroes are abundant around the world, making the source of "Jack the Giant Killer" difficult to pin down.  However, the ascription of Jack's relation to Cornwall suggests a Brythonic (Celtic) origin. The early Welsh tale How Culhwch won Olwen (tentatively dated to c. 1100), set in Arthurian Britain places Arthur as chief among the kings of Britain. The young hero Culhwch ap Cilydd makes his way to his cousin Arthur's court at Celliwig in Cornwall where he demands Olwen as his bride; the beautiful daughter of the giant Ysbaddaden Ben Cawr ('Chief of Giants'). The Giant sets a series of impossible tasks which Arthur's champions Bedwyr and Cai are honour-bound to fulfill before Olwen is released to the lad; and the Giant King must die. Folklorists Iona and Peter Opie have observed in The Classic Fairy Tales (1974) that "the tenor of Jack's tale, and some of the details of more than one of his tricks with which he outwits the giants, have similarities with Norse mythology." An incident between Thor and the giant Skrymir in the Prose Edda of ca. 1220, they note, resembles the incident between Jack and the stomach-slashing Welsh giant. The Opies further note that the Swedish tale of "The Herd-boy and the Giant" shows similarities to the same incident, and "shares an ancestor" with the Grimms's "The Valiant Little Tailor", a tale with wide distribution. According to the Opies, Jack's magical accessories – the cap of knowledge, the cloak of invisibility, the magic sword, and the shoes of swiftness – could have been borrowed from the tale of Tom Thumb or from Norse mythology, however older analogues in British Celtic lore such as Y Mabinogi and the tales of Gwyn ap Nudd, cognate with the Irish Fionn mac Cumhaill, suggest that these represent attributes of the earlier Celtic gods such as the shoes associated with triple-headed Lugus; Welsh Lleu Llaw Gyffes of the Fourth Branch, Arthur's invincible sword Caledfwlch and his Mantle of Invisibility Gwenn one of the Thirteen Treasures of the Island of Britain mentioned in two of the branches; or the similar cloak of Caswallawn in the Second Branch. Another parallel is the Greek demigod Perseus, who was given a magic sword, the winged sandals of Hermes and the 'cap of darkness' (borrowed from Hades) to slay the gorgon Medusa.  Ruth B. Bottigheimer observes in The Oxford Companion to Fairy Tales that Jack's final adventure with Galigantus was influenced by the "magical devices" of French fairy tales. The Opies conclude that analogues from around the world "offer no surety of Jack's antiquity."

The Opies note that tales of giants were long known in Britain. King Arthur's encounter with the giant of St Michael's Mount  – or Mont Saint-Michel in Brittany – was related by Geoffrey of Monmouth in Historia Regum Britanniae in 1136, and published by Sir Thomas Malory in 1485 in the fifth chapter of the fifth book of Le Morte d'Arthur:
Then came to [King Arthur] an husbandman ... and told him how there was ... a great giant which had slain, murdered and devoured much people of the country ... [Arthur journeyed to the Mount, discovered the giant roasting dead children,] ... and hailed him, saying ... [A]rise and dress thee, thou glutton, for this day shalt thou die of my hand. Then the glutton anon started up, and took a great club in his hand, and smote at the king that his coronal fell to the earth. And the king hit him again that he carved his belly and cut off his genitours, that his guts and his entrails fell down to the ground. Then the giant threw away his club, and caught the king in his arms that he crushed his ribs ... And then Arthur weltered and wrung, that he was other while under and another time above. And so weltering and wallowing they rolled down the hill till they came to the sea mark, and ever as they so weltered Arthur smote him with his dagger.
Anthropophagic giants are mentioned in The Complaynt of Scotland in 1549, the Opies note, and, in King Lear of 1605, they indicate, Shakespeare alludes to the Fee-fi-fo-fum chant (" ... fie, foh, and fumme, / I 
Jack the Giant Killer