watertight

adjective2

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. — 1847
2. •
   (figurative) So devised or planned as to be impossible to defeat, evade or nullify.

   • 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. — 2016