The phrase ‘Loose Cannon’ is used to indicate that a person or thing is unpredictable and liable to cause damage if not kept in check.
Example of Use: “Watch out for Justin. He’s a loose cannon, especially when he’s in a bad mood.”
Between the 17th and 19th centuries, wooden warships had cannons mounted on wheels on their gundecks. The cannons had to be kept tied down to prevent them from getting away, and a 'loose cannon' was a gun that had broken free of its restraints and was rolling about the deck in a dangerous way. The term was first seen in its figurative sense in an article published in the 1899 paper, The Galveston Daily News: “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.””