Here is a common phrase that had and has an alternate meaning that relates to the origin of the phrase.
The most common way that the phrase is used is to refer to being totally healthy and free of disease. A person who goes to the doctor for a full examination and who is found to be healthy is said to have a ‘clean bill of health’.
The phrase has nothing to do with a request for payment, as in a doctor’s bill that will need to be paid if you visit a doctor in order to be declared healthy. It does have to do with history, though.
You see, in the past, it wasn’t unusual for plagues and diseases to be spread to the local population of coastal towns when a ship would arrive. If a person on that ship had an illness and came ashore, they could potentially infect anyone they came in contact with.
The sicknesses would be carried from one port to another since it was nearly certain that those people on the ship couldn’t have been infected when they were at sea. At sea, the ship would have been isolated.
Since some illnesses, like the plague, could be devastating to the population of people in the port town, before a ship was allowed to leave a port, they were thoroughly checked for sickness and disease. If the medical examiners found that everyone was in good health. the ship was given a document that stated as much. This document could then be shown to officials at the next port of call, to show that everyone on board was healthy, after which the people on the ship would be allowed to disembark.
The document that declared the people on the ship safe and free of disease was called the Bill of Health.
The phrase is now used even far from any seaport, but the meaning is the same, even if there is no actual document involved. A person with a clean bill of health has been found healthy and free of disease. However, ships are still issued a clean bill of health before they leave a harbor.