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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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impregnable, conceptive | capable of conceiving | |
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impregnable, inexpugnable | incapable of being overcome, challenged or refuted | |
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impregnable, inviolable, unassailable, strong, secure, unattackable | immune to attack; incapable of being tampered with |