Origin of "Loose Cannon"

An unpredictable person or thing, liable to cause damage if not kept in check by others.

Origin:
Arming ships with heavy guns was a hazardous business, even if the vessel was not in battle. The weight of the cannons would affect the steering of a ship and make it difficult to control. But even worse, if the ship hit a storm, and the cannons were not properly secured, they would slide around, knocking holes in the hull. And of course during the battle the cannons needed to be well secured, otherwise, when they were fired, the recoil would send then shooting backwards, causing more damage.