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wa‧ter‧tight

1 a watertight container, roof, door etc does not allow water to get in or out [↪ airtight]
2 an argument, plan etc that is watertight is made very carefully so that people cannot find any mistakes in it
Lucky for him, his alibi is watertight.
Unless the ban is watertight, EU laws will overturn it.
Lucky for him, his alibi is watertight.
Unless the ban is watertight, EU laws will overturn it.