/ ˈwɔːtətaɪt; NAmE ˈwɔːtərt-; ˈwɑːt-/ adjective
1.
that does not allow water to get in or out :
a watertight container
2.
( of an excuse, a plan, an argument, etc. ) carefully prepared so that it contains no mistakes, faults or weaknesses :
a watertight alibi
The case has to be made watertight.