The story
From coco + yam, because old cocoyam was planted in coconut groves and resembled yams. New cocoyam was so named because it resembled old cocoyam.
Etymology from Wiktionary, CC BY-SA 4.0.
cocoyam
Today it means: New cocoyam: Xanthosoma, particularly Xanthosoma sagittifolium, or the edible root of that plant; malanga. Full definition →