https://www.merriam-webster.com/dictionary/corn

Check out Definition 3:

British : the grain of a cereal grass that is the primary crop of a region (such as wheat in Britain and oats in Scotland and Ireland); also : a plant that produces corn

The "corn" we know in America (which is also known as maize) got its name from that usage of the word "corn".

As a side light to the above, Korn is the word for "grain" in German. I believe our corn is sometimes called "sweet corn" in Britain. I've heard stories about American corn getting shipped to Germany after the war in response to a request for 'Korn' and then not knowing what to do with what arrived. Corn is still usually thought of as livestock food.

My guess is the root for corn means grain in general.

In British English 'corn' is used to refer to wheat, barley and other grain crops. Maize is also referred to as 'corn' or 'sweetcorn' in UK English. When UK prime minister Theresa May talked about running through cornfields as a kid recently she meant wheat or barley fields not maize.