A medieval armor piece; a steel or iron reinforcement worn on the arms or shoulders, part of plate armor.
From Old French or Middle English armor terminology, possibly related to Latin 'aternum' or French 'attren,' referring to reinforced armor pieces.
An atren was a specific piece of plate armor that knights wore — if you've seen pictures of knights in full metal suits, the atrens are those reinforcing pieces over the shoulders and upper arms!
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