Bill of Health
A clean bill of health. A document, duly signed by the proper
authorities, to certify that when the ship set sail no infectious
disorder existed in the place.
A foul bill of health
is a document to show that the place was suffering from some
infection when the ship set sail. If a captain cannot show a clean
he is supposed to have a foul one.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894