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indomitable, never-say-die, unsubduable, unconquerable, impregnable, inexpugnable, insuperable, insurmountable, invincible, unbeatable, unvanquishable, unsurmountable | impossible to subdue | |
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unconquerable, insuperable | incapable of being surmounted or excelled | |
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insuperable, insurmountable | impossible to surmount |