near Marlborough. An artificial mound, 130 feet high, and covering seven acres of ground. Some say it is where “King Sel” was buried; others, that it is a corruption of Solis-bury (mound of the sun); others, that it is Sel-barrow (great tumulus), in honour of some ancient prince of Britain. The Rev. A. C. Smith is of opinion that it was erected by the Celts about B.C. 1600. There is a natural hill in the same vicinity, called St. Martin's Sell or Sill, in which case sill or sell means seat or throne. These etymologies of Silbury must rest on the authority of those who have suggested them.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894