1. unyielding. Invincible, impregnable, indomitable suggest that which cannot be overcome or mastered. Invincible is applied to that which cannot be conquered in combat or war, or overcome or subdued in any manner: an invincible army; invincible courage. Impregnable is applied to a place or position that cannot be taken by assault or siege, and hence to whatever is proof against attack: an impregnable fortress; impregnable virtue. Indomitable implies having an unyielding spirit, or stubborn persistence in the face of opposition or difficulty: indomitable will.