Watertight means completely sealed so that no water can get in or out. A watertight container keeps your phone dry even if you drop it in a pool. Submarines need watertight hulls so the crew can travel safely underwater. A watertight roof protects your house during heavy storms.

The word also describes an argument, plan, or excuse that's so well-constructed that nobody can find any flaws or weaknesses in it. When a lawyer presents a watertight case, every piece of evidence fits together perfectly with no gaps or contradictions. If your explanation for why you're late is watertight, it's so logical and complete that no one can question it.

In this second sense, imagine trying to poke holes in a sealed container. A watertight argument is like that container: no matter where someone tries to attack it, they can't find any openings. A student who creates a watertight science fair project has thought through every question a judge might ask. A chess player with a watertight defense has protected every piece so thoroughly that their opponent can't find any way to break through.