I've recently begun reading the Canterbury Tales and in the opening prologue, Chaucer notes that the Miller is a notorious and gleeful thief of his customers' "corn." I thought this seemed strange, since no one in medieval Europe would have seen a cob of corn in their lives.
A quick google search revealed that "Corn" in Old English referred to wheat or any staple cereal crop. So I'm wondering when and why the word stopped meaning "staple cereal crop" and started meaning "that starchy cereal that grows on stalks and tastes great eaten right off the cob with some butter and salt."
'Corn' can still mean 'staple crop' or 'generic grain' in British English, although it's old fashioned it is still in some use today, and we call maize 'sweet corn' to distinguish it from other cereals. So the question really is why did US usage diverge, why don't people in Idaho call wheat 'corn'? Or maybe they do?
Corn had the generic meaning of "grain", or "staple cereal" as you put it. When Europeans came to North America, they encountered Maize, which was new to them. They called it "Indian Corn", because it was cultivated by native Americans. In the late 1700's, white farmers in North America started growing large amounts of Maize themselves (mostly because it could be converted to whisky, for which there was a large market, and which could be easily transported), and the "Indian" part was dropped. Nowadays in the US, we still call multicolored maize "Indian Corn", but the uniformly colored maize is just "corn". Fun fact, well over 90% of the corn grown is not for humans, but for animals.
It didn't. Corn still means the staple crop in each country. In the US, it means maize; in England, it means wheat; in Scotland, it can even mean oats.
Thanks for your answers, everyone! I guess I was overlooking the obvious fact that the name for maize in my region of the United States was not necessarily universal in the English-speaking world!