a.

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

   我要裝上不漏水的窗。

2. （措辭等）嚴密的，無懈可擊的
   His arguments are watertight.

   他的論據非常嚴密。

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.