watertight

adj

1. So tightly made that water cannot enter or escape.

   “What a day, eh? Demmy, it's like the commencement of the rainy season in Bengal. But you'll find my carriage is watertight. Come along, my mother and Emmy are in the vestry.”

   “There is no reference to rough weather, and, indeed, the state of the vessel’s paint and rigging excludes the idea that she was abandoned for any such reason. She is perfectly watertight.”

2. (figuratively)So devised or planned as to be impossible to defeat, evade or nullify.

   “a watertight alibi”

   “a watertight contract”

   “a watertight regulation”

   “While the Nationwide League suffer the consequences of not having negotiated a watertight agreement with ITV Digital, the Premier League can be excused a smug smile of satisfaction over the deal they struck with the fallen digital company.”

   “David Cameron has said his EU deal will be legally binding and watertight, after a senior European politician claimed countries could simply undo it after the UK voted to remain in the union.”