The term "corned beef" comes from the original definition of the word "corn." While we associate the word with maize, it actually stems from the Latin word meaning "grain."
Corn came to describe any seed or grain product in England, Scotland, and Ireland. It actually described different crops based on what was more popular in a given region.
The result was Irish corned beef, named after the "corns" of salt used to preserve the meat. Hence, the name "corned" stuck as a term for any meat preserved this way.