Ex fida bona is a Roman expression and principle of law.
A judge is to make a judgement based on "good business norms". It means that the contractual parties shall keep their words, making it possible for both to trust each other. An agreement should be according to the branch-norms if not specifically mentioned.
This was a condition to permanent trade relations during the rise of Rome. It was during the 2nd century BC that the Roman praetors started using this principle, as commerce grew in the Mediterraneansea.