wa ‧ ter ‧ tight /ˈwɔːtətaɪt $ ˈwɒːtər-, ˈwɑː-/ BrE AmE adjective
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.