Here’s an idiom you hear a lot in the media and politics.
“He’s a real loose cannon.”
There’s a movie titled after it, a musical band by the same name, and if you type “loose cannon” into Google search you’ll come up with 3,320,000 hits using the term for everything from breweries to football coaches.
To define this idiom: A “loose cannon” is a person or thing which is unpredictable, uncontrollable and highly destructive. An example would be: Joe is considered a loose cannon due to his volatile personality and lack of ability to maintain his composure.
Okay, now that we know what it means, where did it come from?
It’s actually a nautical reference to the dangers of an untethered cannon. Something as heavy as an iron cannon rolling hither and yon on choppy seas was, in the very least, destructive, although it’s never been known for sure to have ever been uttered by actual sailors.
The term was actually first alluded to by french author Victor Hugo as he was setting the scene in his novel Ninety Three, 1874 of a cannon being tossed about the deck of a ship following a violent incident onboard. The English translation from the original French reads:
“The cannonade, hurled forward by the pitching, dashed into this knot of men, and crushed four at the first blow; then, flung back and shot out anew by the rolling, it cut in two a fifth poor fellow… The enormous cannon was left alone. She was given up to herself. She was her own mistress, and mistress of the vessel. She could do what she willed with both.”
A year later, Henry Kingsley picked up this reference in his novel Number Seventeen, 1875, in which he made the first use of the actual term ‘loose cannon’ in English:
“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 phrase, although it surfaced a few times after that, almost dwindled into obscurity until US president Theodore Roosevelt inspired its resurrection in the autobiography of long time and close friend William White. Published soon after White’s death in 1944, the book entitled Autobiography contained the following reminiscence:
He [Roosevelt] said: “I don’t want to be the old cannon loose on the deck in the storm”.
The rest, as they say, is history. The term gained in popularity becoming the proverbial loose cannon it is today. You gotta love irony.
The Wurd Turtle 
I loved this. even after 13 years of being married to a sailor this is one thing he never mentioned
I’m glad you enjoyed it!