This interesting and unusual surname is of Anglo-Saxon origin, and is a locational name from Cosby south-west of Leicester in Leicestershire. The placename is recorded as "Cossebi" in the Domesday Book of 1086, and as "Cosseby" in the 1236 Feet of Fines for that county, and derives from the Olde English pre 7th Century personal name "Cossa" (also the first element of Cosham, Hampshire), with the Olde English "by", from the Old Norse "byr", homestead, settlement; hence, "Cossa's homestead". During the Middle Ages, when migration for the purpose of job-seeking was becoming more common, people often took their former village name as a means of identification, thus resulting in a wide dispersal of the name. In the modern idiom the name can be found as Cosby, Cosbey, Cosbee, Cozby, Cowsby, Causby, Causbey and Cawsby. An important Anglo-Irish Cosby family have been associated with County Leix since the early fifteen hundreds. Recordings of the surname from London Church Registers include: the marriage of Elyzabethe Cosbye and John Berden on April 13th 1551, at Christ Church, Greyfriars; the christening of Richard Causby on November 29th 1560 at St. Botolph's, Bishopsgate; and the marriage of Henry Causby and Jane Linsdall at St. Giles' Cripplegate, on November 28th 1601. The first recorded spelling of the family name is shown to be that of Francis Cosby (Irish General), which was dated 1548, in the "Records of Queen's County", Ireland, during the reign of King Edward V1 of England, known as "The Boy King", 1547 - 1553. Surnames became necessary when governments introduced personal taxation. In England this was known as Poll Tax. Throughout the centuries, surnames in every country have continued to "develop" often leading to astonishing variants of the original spelling.