This is an anglicization of the Gaelic name O Caiside - a compound of the elements "O" meaning "grandson" or "male descendant" and the personal name Caiside (coming from "cas" - curly hair). The O'Cassidys belonged to County Fermanagh in the province of Ulster and they provided physicians to the Maguires (a name which means pale-coloured!) between 1300 and 1600. The (O)Cassidys also excelled in the field of literature. One Rory O'Cassidy, Archdeacon of Clogher, is said to have helped compile the 15th century "Annals of Ulster". In the modern idiom the name is also spelt with an initial "k" The first recorded spelling of the family name is shown to be that of Giolla Moduda O'Cassidy (Gaelic poet) which was dated 1143, during the reign of King Stephen, known as "The Count de Blois" 1135 - 1154. 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.