(The). There are three, viz. Stoke, Desborough, and Bonenham (or Burnham). At one time the Chiltern Hills, between Bedford and Hertford, etc., were covered with beech trees which formed shelter for robbers; so a steward was appointed by the Crown to put down these marauders and protect the inhabitants of the neighbourhood from depredations. The necessity of such watch and ward has long since ceased, but the office remains; and, since 1750, when a Member of Parliament wishes to vacate his seat, one way of doing so is by applying for the stewardship of the three Chiltern Hundreds. The application being granted, the Member is advanced to an office under the Crown, and his seat in the House is ex officio vacated. Immediately the Member has effected his object, he resigns his office again. The gift is in the hands of the Chancellor of the Exchequer. It was refused to a Member for Reading in 1842.
The Stewardships used for a similar purpose were Old Sarum (in Sussex), East Hendred (in Berks), the Manor of Poynings (in Sussex), Hempholwic (in Yorkshire), all of which have dropped out of use. The Stewardship of the Manor of Northstead (in Yorks) survives (1894), but the Escheatorships of Munster and Ulster were abolished in 1838.
The London Gazette of August 4, 1893, announced that the “Chancellor of the Exchequer has appointed William Henry Grenfell to be steward and bailiff of the Chiltern Hundreds in the room of John Morrogh, resigned.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Chiltern Hundreds from Infoplease: