We know what the ‘wall’ bit means for definite. It has the exact same etymology as Wales, which is possibly slightly better known than the ‘wall’ bit of Cornwall.
It is an English word – or, I should say, Old English, Anglo-Saxon word – that was given to these two places that we now know as Cornwall and Wales. This word was Wēalas, and it sometimes gets translated as ‘foreigners’ or ‘strangers’, but the best way to think of it is just “not us”. Them. Other.
This is what the English name ‘Cornwall’ actually means – otherness. Its separateness from England – or certainly Anglo-Saxon territory – is really explicit in the name (just as it is in the name Wales).
Wales and Cornwall both had their own names too – Wales we know as Cymru, and Cornwall in modern Cornish is Kernow. But in English, they both became Wēalas.
The ‘corn’ bit of Cornwall is probably from that local name. We’re not exactly sure where that corn bit actually means, but it's probably got an etymological link to cornu, which would be a Latin word in origin, meaning horn. If you think about what Cornwall looks like, it juts out into the Atlantic, so ‘corn’ probably has something to do with the form of the place, a horn-like peninsula sticking out. So there you have it: Cornwall means something like ‘the un-English others of the horn-place’.
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Tim Hannigan is the author of The Granite Kingdom: A Cornish Journey (Apollo, 2023). Listen to him talk about the history of Cornwall on the HistoryExtra podcast