Now there's a legal phrase whose time has come -- and gone. Apparently it's common in admiralty law, and it means "of the utmost good faith," as in "Marine insurance is a contract 'uberrimae fidei,' requiring the utmost good faith by both parties to the contract." I've been in law teaching for nearly 14 years, and had never heard of it until today. Use it at your own peril, unless you're using it in a field where it's firmly established.
Uberrimae Fidei, Back in the News:
We consider the doctrine that's on everyone's lips: uberrimae fidei.
For more on this, see here. Thanks to James S. Tyre for the pointer.