Sealed so well that water cannot get in or out; also used to mean a plan or argument that has no flaws or weaknesses.
Compound of 'water' (from Old English 'wæter') and 'tight' (from Middle English, meaning firmly fixed). First used literally for ships and containers in the 1800s, then extended metaphorically to mean 'airtight' or 'foolproof.'
Naval engineers invented this term to describe ship compartments that could survive sinking, but it became one of the best metaphors in English for an argument you absolutely cannot poke holes in—which is why lawyers and debate champions love it.
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