Unsound means not solid, reliable, or based on good reasoning. When something is unsound, you can't trust it to hold up under pressure or examination.

A building with cracks in its foundation is structurally unsound: it might look fine from the outside, but it could collapse. An old bridge might be declared unsound and closed until repairs make it safe again. In these cases, unsound means physically weak or damaged.

The word also describes faulty thinking or poor logic. An unsound argument might seem convincing at first, but it relies on false assumptions or logical errors. If someone claims that because their lucky socks helped them win one game, wearing those socks will always guarantee victory, that's unsound reasoning. It confuses coincidence with cause and effect.

In contrast, sound means solid, reliable, and well-constructed. A sound argument is built on facts and logic. Sound advice comes from careful thought and experience. When you hear that something is unsound, whether it's a theory, a plan, or a structure, it means you should examine it carefully before trusting it. The word warns you that what looks stable on the surface might not hold up when tested.