according to English mythology, is a corruption of Debon's-share. This Debon was one of the heroes who came with Brute from Troy. One of the giants that he slew in the south coasts of England was Coulin, whom he chased to a vast pit eight leagues across. The monster trying to leap this pit, fell backwards, and lost his life in the chasm. When Brutus allotted out the island, this portion became Debon's-share.
And eke that ample pit, yet far renowned For the large leap which Debon did compell Coulin to make, being eight lugs of grownd, Into the which retourning back he fell ... In mede of these great conquests by them got Corineus had that province utmost west ... And Debon's share was that is Devonshire
Spenser: Faërie Queene, book ii. canto x. 11, 12.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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