Jacks are nimble. Jacks are quick. Jacks do not get caught in traps. Jacks kill Giants. The word was originally (and obviously still is) a nickname for John, an extremely popular given name; and from as early as the 13th century it was used to describe a representative member of the common folk (> Estates Satire). But in English mystery plays after about the beginning of the 15th century the character named "Jack" represented Everyman-as-survivor, like the Jack who tricks the Green Man of Knowledge in the oral Fairytale, stealing his daughter from Faerie; and it is clear from this usage that, from early on, the name had wider implications than simply the identification of a commoner with a good ration of mother wit.
Saucy and pert and handy, but capable of cruelty, the Jack is a figure, like Robin Hood, who almost certainly embodies echoes of pre-Christian myths. He is a wise Fool, a Trickster. This halo of the chthonic, which is exceedingly difficult to pin down, may well explain the allure of the various Jack figures in innumerable rhymes and fairytales: the Jack who climbs the Beanstalk and rifles the treasure of the Giant; the Jack whose bargains, each of them magical, gains him the king's daughter; Jack the Giant-Killer, whose four Magic possessions make him into a Shapeshifter; Jack Sprat, who is a Dwarf; Jack Frost and Jack Horner and Jack Ketch and Spring-Heeled Jack, the latter capable of jumping over rooftops; Jack the Ripper, so-dubbed on the assumption that he was a trickster figure rather than someone mentally ill. Jack is both a figure of daylight trickery and a creature of the shadows, a Trompe-L'oeil flicker at the edge of Perception: man or animal or both. He patrols Thresholds, but does not own the Land, for he is not a Hidden Monarch (a Jack of All Trades is likely a Master of None); he is Jack in the Green – the wickerwork image of the Green Man who is paraded through the village on Mayday and whose ritual death signals the annual renewal of the vegetable world (> Golden Bough). He is Jack o' Lantern, the ignis fatuus or Will o' the Wisp, the illusory flame which draws folk into marshy ground, a trickster psychopomp who leads dreamers out of their depth, and may abandon them there.
Jack figures appear variously in fantasy texts, under various appellations. Jack Pumpkinhead figures in the Oz books by L Frank Baum. In Michael Tippett's Opera The Midsummer Marriage (1955) Jack, the Papageno-like mechanic, disguises himself as a Sibyl and is comically unmasked. The eponymous trickster in Roger Zelazny's Jack of Shadows (1971) is significantly an amoral prowler of the twilight between Dayside and Darkside, until he begins to grow up. Charles de Lint's Jack of Kinrowan (omni 1995) unusually treats the Jack (in this case a female) as protagonist. The Walker of Worlds in Tom de Haven's Chronicles of the King's Tramp sequence (1988-1992) is a Jack by name and nature. Jack Mythagoes patrol the wild wood in Robert Holdstock's continuing Mythago sequence. [JC]