The surname Cornish was well established by the 1500s. Its presence outside Cornwall would be unsurprising, However, what requires more explanation is the considerable number of people in Cornwall itself with this name. Their presence in the 1500s implies its original meaning was not ‘someone from Cornwall’ as in Cornwall this would not be a very good discriminator.
The name was mainly based in eastern parts, in the English-speaking area of Cornwall, although significant clusters were also found in mid-Cornwall around St Austell and St Columb and the odd parish register entry even further west. Its eastern bias suggests first that this second name had become hereditary at an earlier point, probably well before 1400. Second, it may have originated at a time when the Cornish language was on its last legs in the east and when the nickname Cornish (in English note) for someone who was still speaking Cornish had become a useful discriminator. Alternatively, or in addition, it could have been a nickname given to someone with pronounced ‘Cornish’ characteristics, whatever they were at the time.
Tellingly, we find nobody in Cornwall in the 1500s named English. This might imply that the English-speaking Cornish in medieval times regarded themselves as de facto English, as they shared the same language.
However, this was not the case in the west. Here, while the surname Cornish was rare, and may have been present only as a result of movement from the east, there were considerably more people in the 1500s named Syse, Says, Size or similar. This was the Cornish-language word for ‘English’ and was presumably given to someone from England or someone who was an English speaker. Its distribution in the 1500s was confined entirely to the Cornish-speaking districts, being found no further east than St Dennis and St Columb.
Perhaps Syse was not in the 1500s a fixed, permanent surname, but more like the appellation Briton/Breton, Frenchman or Irish given to migrants from overseas. Evidence for this might be the sharply reduced number with this name in the mid-1600s. By 1641/42 there were just six men named Sise/Size/Sayse in Cornwall. Contrast that with the 63 named Cornish. Sise/Size survived, but only just, in contrast to the English name Cornish, which flourished, a metaphor with wider implications perhaps.