Pantherophis guttatus eats rodents, birds and eggs, and takes no interest in grain. Farmers in the southeastern United States nonetheless found the snake repeatedly in and around corn cribs, because stored maize attracted mice and rats, and the mice and rats attracted the snake. The association gave the animal its name and, historically, a degree of tolerance from the people whose stores it protected.
A second derivation is often cited alongside it: the chequered black-and-white belly resembles the patterning of Indian corn. Both explanations appear in the older literature and neither can now be shown to be the original.
The more consequential confusion is with the copperhead. Corn snakes are frequently killed on sight because their reddish blotches read as venomous. The distinguishing marks are reliable: the corn snake has round pupils, a spear-shaped mark on the top of the head, and squarish dorsal blotches, while the copperhead has vertical pupils, a heat-sensing pit between eye and nostril, and hourglass bands that narrow along the spine.