The surname of SIDDENHAM was a locational name 'of Sydenham' a parish in County Kent, near Blackheath. Early records of the name mention John le Sydenham, County Somerset, 1273. William de Sydenham of Yorkshire, was listed in the Yorkshire Poll Tax of 1379. Simon de Sidenham, 1300, ibid. Humphrey Sydenham and Elizabeth St. John were married in Canterbury, Kent in the year 1674. Thomas Sydenham and Frances Bunbury were married at St. George's, Hanover Square, London in the year 1803. A family of this name, originally from Sydenham in Somerset, settled at Wynford Eagle in County Dorset. This estate had been bought in the time of Henry VIII. Descendants included Thomas Sydenham (1624-89) who is regarded as the founder of clinical medicine. Originally the coat of arms identified the wearer, either in battle or in tournaments. Completely covered in body and facial armour the knight could be spotted and known by the insignia painted on his shield, and embroidered on his surcoat, the draped garment which enveloped him. Between the 11th and 15th centuries it became customary for surnames to be assumed in Europe, but were not commonplace in England or Scotland before the Norman Conquest of 1066. They are to be found in the Domesday Book of 1086. Those of gentler blood assumed surnames at this time, but it was not until the reign of Edward II (1307-1327) that second names became general practice for all people. The rise of surnames, according to the accepted theory, was due to the Norman Conquest of 1066 when Old English personal-names were rapidly superseded by the new christian names introduced by the Normans. Of these, only a few were really popular and in the 12th century this scarcity of christian names led to the increasing use of surnames to distinguish the numerous individuals of the same name. Some Normans had hereditary surnames before they came to England, but there is evidence that surnames would have developed in England even had there been no Norman Conquest. The development of the feudal system made it essential that the king should know exactly what service each person owed. Payments to and by the exchequer required that debtors and creditors should be particularized, and it became official that each individual acquired exact identification.
The associated arms are recorded in Sir Bernard Burkes General Armory. Ulster King of Arms in 1884.