I was always taught that the expression "loose cannon" originated from shipboard blackpowder cannons (think the cannons you see in movies of pirate ships, etc) which were attached to wooden trolleys, and the trolleys tethered to the porthole structure. If the cannon came out of the trolley, or the trolley became unattached to the ship structure, it could roll around in hull, and do damage to the ship, the crew, and goods.
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