History does funny things to names and this is a good example of Medieval creativity. The derivation is from the early greek female personal name "Cassandra". "Cassandra" appears to have been a "Crusader" import, which is perhaps surprising as it translates as "the betrayer of men!". In this case we have an example of a diminutive "Cass" plus the French "petit", to give "little cass", although this was immediately concertinered to "Cass-et". Early recordings include examples with the intrusive "N", introduced to aid pronunciation as in the recording John Causnet. Christened at the church of St. Botolph without Aldgate, London on February 5th 1660, in the year of the restoration of Charles 11 1660-1685. The first recorded spelling of the family name is shown to be that of Samuel Casnet, which was dated May 15th 1633, christened at St. Anne's Church, Blackfriars, London, during the reign of King Charles 1, "The Martyr", 1625 - 1649. 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.