E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
according to English mythology, is a corruption of Debons-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 Debons-share.
And eke that ample pit, yet far renownea
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 Debons share was that is Devosshire.
Spenser: Faërie Queene, book ii. canto x. 11, 12.