'Corn' once meant any grain at all — it became 'maize' only after English colonists hit the New World.
(Originally) any cereal grain or the plants producing it; (in North America) maize, specifically Zea mays.
From Old English 'corn' (grain, seed, a single grain, cereal plant), from Proto-Germanic *kurną (grain), from PIE *ǵr̥h₂nóm (worn-down grain, mature grain), from *ǵerh₂- (to grow old, to mature, to ripen). Originally 'corn' meant any grain whatsoever — wheat in England, oats in Scotland, rye in northern Germany. When English settlers arrived in America, they applied the word to the dominant local grain, maize, and this specialized meaning
In the King James Bible (1611), 'corn' means wheat or barley — not maize, which was unknown in the ancient Near East. When Genesis describes 'corn in Egypt,' it means grain. 'Corn' only became synonymous with maize in American English because colonists applied their generic word for grain to the new crop the Native Americans