‘Of the utmost good faith’ (contrast with bona fide). Certain contracts may be void or voidable unless the party complained of has acted in the utmost good faith, i.e. has disclosed all facts relevant to the contract whether or not he has been asked to do so. The most common are contracts of insurance and partnership: a party to be insured must tell the insurer everything that may affect his decision in issuing a contract. Again, a partner must be quite open in his dealings and must not make secret profits, etc.
Reference: The Penguin Business Dictionary, 3rd edt.