an airtight
Examples
I don't really deeply feel that anyone needs an airtight reason for quoting from the works of writers he loves, but it's always nice, I'll grant you, if he has one.
Criticism always seemed to me a lot like police work. You look for clues, fingerprints, motives. You need to construct an airtight case.
When George W. Bush and his foot soldiers can't build an airtight legal case, suspicion and xenophobia will suffice.