The surname Sillworthey was first found in Somerset at Selwood which dates back to c. 894 where it was listed as Seluudu and probably meant "wood where swallow-trees grow." 1
The ancient Selwood Forest ran approximately between Gillingham in Dorset and Chippenham in Wiltshire. Ælfgar of Selwood was a saint venerated at a chapel in the forest of Selwood.
One source notes the name denotes a "dweller at Selwood (the forest on the borders of Wilts and Somerset), the Anglo-Saxon Seal wudu (Anglo- Saxon Chronicles, A.D. 894), Seal wydu (A.D. 878) = apparently the 'Willow-Wood' [Old English seal, a willow or sallow]; but Asser, in his Life of Ælfred, translated Seluudu as Silva Magna in Latin and Coit Maur (mod. Coed Mawr) in Welsh, i.e. 'Great Wood,' as if the first element of the Anglo-Saxon wood-name were the Old English sél, ' good,' and its meaning could be extended to signify 'great.'" 2
Alternatively, the name could have derived from Selworthy, also in Somerset, a small village and civil parish in the hundred of Carhampton which dates back to the Domesday Book where it was listed as Seleuurde and literally meant "enclosure or settlement near sallow-trees" from the Old English sele + worth 1. At that time the lands were held by Ralph de Limesy.
Early feudal rolls provided the king of the time a method of cataloguing holdings for taxation, but today they provide a glimpse into the wide surname spellings in use at that time. John Selewode was recorded in 1189 and later Richard de Selwode was listed in London in 1339. 3
England
