It has been suggested that the family derived its name from Sai, about two miles SE of Argentin in Normandy, though there is no substantial evidence for this. Certainly William de Say, possibly a Norman, married by 1098 Agnes, daughter of Hugh de Grentmesnil.
But the known pedigree originates with another William de Say, granted his (probably dead) father's lands in a charter of the Empress Maud between Christmas 1141 and June 1142; joined his brother-in-law Geoffrey de Mandeville, 1st Earl of Essex of the 1140 creation, in rebellion against Stephen 1144; married Beatrice (died 19 April 1197 or some year before it), daughter of William de Mandeville, sister of 1st Earl of Essex and divorced wife of Hugh Talebot, and was allegedly killed in an attack on Burwell Castle, Cambs, along with his brother-in-law, 1st Earl of Essex Aug 1144, though he may well have died later. [Burke's Peerage]
Note: Magna Charta Sureties has Geoffrey de Mandeville dead 14 Sep 1144. Also note that I have this William as a grandson of William de Say & Agnes de Grentmesnil.
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William de Say I is chiefly known as an associate of Geoffrey de Mandeville, 1st Earl of Essex, in his last revolt, with Hugh Bigod, against Stephen in 1144. In the 2nd charter of the Empress Maud to the Earl, issued between Christmas 1141 and the end of June 1142, William was given a special grant of his father's lands. According to Henry of Huntingdon, he was killed with the Earl in Aug. 1144 when, after occupying Ramsey Abbey, they were attacking Stephen's fort at Burwell, Cambs. but there is evidence that William surv. the Earl (f). He m. Beatrice, divorced wife of Hugh Talebot, sister of Earl Geoffrey above named, and da. of William de Mandeville. The Earl is said to have brought her to England for the marriage. She lived to a great age; d. 19 Apr, in or before 1197, and was buried at Walden Abbey. (Complete Peerage XI:464-5 (transcribed by Thomas Aiken))
(f) The Earl himself did no die until Sep. Dr. G Herbert Fowler, in a paper on the Beauchamps, Barons of Eaton, cited a confirmation to St. Neot's Priory by William de Say and Beatrice his wife and William his heir for the soul of Earl Geoffrey de Mandeville of a grant of land in Eaton Socon, Beds. The original grant was made for the soul of the same Earl by Hugh de Beauchamp, who, Dr. Fowler says, was William's man in respect of the land. Eaton Socon had been restored to the Earl in 1142 by the Empress Maud, and such interest as William possessed must have been in right of his wife. The "Libellus de Anniversariis of Ramsey Abbey" says that William de Say d. 11 Jan without giving the year. This refers to William de Say I because William II d. about Aug in 1177.
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William de Say, son of William de Say, and grandson of William de Say, who came into England with the Conqueror. He m. Beatrix, the divorced wife of Hugh Talbot, and dau. of Geoffrey de Mandeville, Earl of Essex, sister of Godfrey, and aunt, and eventually heiress of William de Mandeville, Earls of Essex, by whom he had issue, William and Geoffrey. [Sir Bernard Burke, Dormant, Abeyant, Forfeited, and Extinct Peerages, Burke's Peerage, Ltd., London, 1883, p. 476, Saye, or Say, Barons Saye]