Meaning of “watertight” - Learner’s Dictionary

watertight

adjective uk us /ˈwɔːtətaɪt/
WATER

Something that is watertight prevents any water from entering it.

REASON/EXCUSE

A watertight reason or excuse is one that no one can prove is false:

a watertight alibi

(Definition of “watertight” from the Cambridge Learner’s Dictionary © Cambridge University Press)