a.

1. 不透水的，防水的
   I'll have watertight windows fixed.

   我要装上不漏水的窗。

2. （措辞等）严密的，无懈可击的
   His arguments are watertight.

   他的论据非常严密。

adj.

1. closely sealed, fastened, or fitted so as to prevent the passage of water.
2. (of an argument or account) unable to be disputed or questioned.