The point where something fails to work properly or to hold up is its weakness. If a building crumbles in a storm, it's probably because of a weakness — a flaw or defect — in its structure.
Weakness can indicate a physical weakness, like a chair with one wobbly leg, or flaw in an idea, like a plan for a lemonade stand that doesn't include any cups for serving the lemonade. People can show weakness too, when they are helpless to take action. The origin of weakness can be traced all the way back to its Proto-Indo-European root of weik, "to bend."