watertight
adj.adj. describing something that does not let any water through. It can also describe an argument or plan that is so good that there are no mistakes or holes in it.
adj. impermeable to water; also used figuratively to describe an argument or plan that is logically sound and free from flaws. Often used predicatively after a linking verb or attributively before a noun.
The new tent is completely watertight.
The lawyer presented a watertight defense that left the jury with no questions about the client's innocence.
While the physical seals on the container were watertight, the logical gaps in the company's financial report made the entire proposal vulnerable to scrutiny.
From water + tight. Compare Scots wattirticht (“watertight”), West Frisian wetterticht (“waterproof”), Dutch waterdicht (“waterproof, watertight”), German wasserdicht (“watertight, waterproof”), Danish vandtæt (“watertight”), Swedish vattentät (“watertight, waterproof”).