This name is of Olde Welsh origin and derives from "Cors" meaning a moor or bog. Originally this would have been locational i.e. "the dweller by the moor", however, in the Middle Ages it developed as a patronymic to mean the "son of Cors" or "little Cors" from the French "petit". The "r" seems to have been dropped from the name by the 18th century although in the meantime other variant spellings such as Cosslett, Coslitt and Caslett were developed. Although the name is found in other regions it appears to be most prevalent in the Glamorgan area. The first recorded spelling of the family name is shown to be that of Margaret Coslett which was dated 1754 Christened at Llys-Faen, Glamorgan. during the reign of King George 11, known as "The Last Warrior King" 1727 - 1760 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.