Impregnable describes something so strong or secure that it cannot be captured, broken into, or defeated. A castle with towering walls, deep moats, and heavily guarded gates might be called impregnable because no army could possibly breach its defenses. A safe with foot-thick steel walls and complex locks could be impregnable.

Think of medieval fortresses built on high cliffs with multiple walls: attackers would struggle to even reach them, let alone break through their defenses.

You'll also hear impregnable used figuratively. A debater with an impregnable argument has thought through every possible objection and built a case that opponents can't tear down. A goalie who saves every shot might seem impregnable during a championship game. When a scientist develops an impregnable theory, it means the evidence supporting it is so strong that critics can't find flaws in it.

The word suggests more than just strength: it implies that something has been tested or could be tested and would still hold firm. Calling something impregnable means it would take extraordinary effort, or perhaps even be impossible, to overcome its defenses.