Figuratively, a loose cannon is an unpredictable or uncontrolled person who is liable to cause unintentional damage.
But in practice, it was one inadequately lashed in place on the deck of a ship, which caused havoc by rolling dangerously and unpredictably.
The first known mention of a loose cannon being tossed about the deck of a ship is in Quatre-vingt-treize (spelt Quatrevingt-treize by the author), the last novel, published in 1874, by the French poet, novelist and playwright Victor Hugo (1802-85). This book is about the counter-revolutionary uprisings which took place in north-western France in 1793. On board an English ship, the Claymore, a group of royalists are planning to land the Marquis de Lantenac, a Breton aristocrat whose leadership could transform the fortunes of the rebellion. But a gunner has failed to properly secure his cannon, which rolls out of control and damages the ship. While describing the accident, Hugo wrote:
Vous pouvez raisonner un dogue, étonner un taureau, fasciner un boa, effrayer un tigre, attendrir un lion ; aucune ressource avec ce monstre, un canon lâché. Vous ne pouvez pas le tuer, il est mort ; en en même temps, il vit. Il vit d’une vie sinistre qui lui vient de l’infini.
In Ninety-three, the 1874 translation by Frank Lee Benedict (1834-1910), this passage is as follows:
You can make a mastiff hear reason, astound a bull, fascinate a boa, frighten a tiger, soften a lion; but there is no resource with that monster, a cannon let loose. You can not kill it—it is dead; at the same time it lives. It lives with a sinister life bestowed on it by Infinity.
As early as 1875, in Number Seventeen, the English novelist Henry Kingsley (1830-76) used the expression as we now know it, but again in its literal sense:
At once, of course, the ship was in the trough of the sea, a more fearfully dangerous engine of destruction than Mr. Victor Hugo’s celebrated loose cannon.
The earliest recorded use of loose cannon in its figurative sense occurs in the following from The Galveston Daily News (Texas) of Thursday 19th December 1889:
Intelligence Always Supreme.
What the southern whites should learn is that, in order to preserve the supremacy of the intelligent class, it is not necessary that all the intelligent voters should be banded together against the voters who are ignorant. A division of the intelligent vote into two great parties would involve a corresponding division of the ignorant vote; for ignorance can never lead, and the ignorant population could not maintain itself as a political unit. He who thinks that in any large community the ignorant vote holds the balance of power, fails to take account of the intelligent vote and of the fact that the votes of ignorant men are, almost invariably, cast in obedience to the command or advice of an intelligent leader. The negro vote in the south is a unit now mainly because it is opposed by the combined white vote. It would in no event become, as Mr. Grady once said, “a loose cannon in a storm-tossed ship,” for the very reason that it has not intelligence enough to voluntarily stand alone as a class and vote as a political unit. [Denver Republican.