(London). So called from a family mansion of the Sydneys, Earls of Leicester, which stood on the north-east side.
“The Earl of Leicester, father of Algernon Sidney the patriot ... built for himself a stately house at the north-east corner of a square plot of `Lammas Land.' belonging to the parish of St. Martin's which plot henceforth became known to Londoners as Leicester Fields. A square gradually grew up on the spot, and was completed in 1671.” —Cassell's Magazine, London Legends, x
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894