This is an idiom that is used a lot by newscasters and commentators in the course of politics, especially during bouts of great national upheaval and times of potential danger. Like now.
Merriam-Webster’s online dictionary provides a succinct definition, saying that a loose cannon is “… a dangerously uncontrollable person or thing,” which nails it, all right.
However, an interesting aspect of this idiom is that there is not necessarily a sense of either willfulness nor innocence attached to the phrase. Whether someone is a deliberate mischief-maker or acts on pure id is beside the point. First and foremost is what that person is, and that is a loose cannon. Unpredictable. Unstoppable. Something terrifying indeed.
First, Some Naval Background
To understand the origin of the expression a loose cannon, me hearties, first a bit of lore regarding the age of sail, a time lasting from the 17th to mid-19th century. During this time, large square-sailed ships with massive timbers and batteries of heavy cannons ruled the waves. (Think “Old Iron Sides,” “HMS Victory,” and Master and Commander.)
The guns on these vessels were lined up on both sides of the warships and often on more than one deck. This was done in order to provide massive directed fire, called a broadside, to withering effect on opposing ships. To view a sea battle two or three hundred years ago was to witness a massive slug fest among majestic, slow-moving ships.
Running cannons inboard for loading, and then pulling them outboard for aiming and firing required a lot of heavy heave-hoing on the part of highly-trained gun crews. The gunnery sailors would haul on tackle that consisted of ropes run through an arrangement of heavy block pulleys. This tackle, running on both sides of a gun, was secured to the ship’s bulwark and then to a stout four-wheeled carriage that cradled each cannon.
Sometimes, during the heat of battle or due to material fatigue, both of these riggings as well as the thick breach line (used to break the recoil of a fired gun) would snap.
And then all hell would break loose, with as much as 3,400 pounds of cast bronze or iron rolling wildly on deck, pushed back and forth by the action of the sea. A loose cannon would smash into other cannons, crew members, masts and sometimes even punch a hole in a ship’s side.
Hence the disconcerting reality behind the phrase a loose cannon, and why that figure of speech is still so evocative.
Literary Origin of the Idiom
It should be noted however that the expression a loose cannon itself did not emerge from the experiences of sailors, but instead from the imagination of Victor Hugo, author of Les Misérables and The Hunchback of Notre-Dame, as well as a long list of other historical works of fiction, according to the website The Phrase Finder.
In this excerpt from his 1874 novel Quatrevingt-treize (Ninety-Three), Hugo asks “How to control this enormous brute of bronze? … How to fetter this monstrous mechanism for wrecking a ship? … The horrible cannon [free of restraining ropes] flings itself about, advances, recoils, strikes to the right, strikes to the left … crushes men like flies.”
A year later, British writer Henry Kingsley in his novel Number Seventeen, mentioned loose cannon in English for the first time while crediting Hugo:
“At once, … the ship was in the trough of the sea, a more fearfully dangerous engine of destruction than Mr. Victor Hugo’s celebrated loose cannon.”
By the end of the 19th century the expression a loose cannon began to be used figuratively with the meaning it has today. And the rest, as they say, is etymology.