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原比喻防御十分坚固，不可摧毁。也比喻团结一致。
[bastion of iron,impregnable fortress as a wall of bronze;impregnable like walls of brass and iron] 亦作“铁壁铜墙”,比喻防御工事坚固,也比喻力量的强大、意志的坚定
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