Soy is a plant, also called soybean, whose beans are used to make foods like tofu, soy milk, and soy sauce. The word can also describe products made from this plant.
From Japanese *shōyu* (soy sauce), which was borrowed into Dutch as *soja* and then into English as *soy*. The name originally referred to the sauce, then broadened to the bean itself.
English met soy through the sauce first, not the bean—so the condiment’s name ended up standing for the whole plant. It’s like learning about “ketchup” before learning about “tomatoes,” then calling the tomato a “ketchup bean.”
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