I've not heard of that idiom, sorry. There's the adjective watertight meaning without flaws or loopholes; "an ironclad contract"; "a watertight alibi"; "a bulletproof argument". Synonyms: bulletproof, ironclad, unshakable, unassailable, incontestable, incontestible - incapable of being contested or disputed.
Source: watertight - definition of watertight by the Free Online Dictionary, Thesaurus and Encyclopedia.
Hope that helps.
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