loose cannon

American  

noun

1. a person whose reckless behavior endangers the efforts or welfare of others.

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loose cannon British  

noun

1. a person or thing that appears to be beyond control and is potentially a source of unintentional damage

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loose cannon Idioms  

1. One who is uncontrolled and therefore a serious and unpredictable danger. For example, We can't trust her to talk to the press—she's a loose cannon. This metaphoric expression alludes to cannon mounted on the deck of a sailing ship, which if dislodged during combat or a storm could cause serious damage to both vessel and crew by sliding about. Its figurative use dates from the first half of the 1900s.

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Usage

What does loose cannon mean? A loose cannon is someone who behaves unpredictably and recklessly, often in a way that puts others at risk or negatively affects what they’re trying to do. People considered loose cannons can’t be trusted because there’s no telling what they’re going to say or do. Loose cannons are thought to be uncontrollable. The metaphor is based on the image of a cannon on a ship that comes loose (such as during a storm) and starts dangerously sliding around and slamming into the people on board and the parts of the ship, causing injuries and damage. Example: I need everyone on my team to be on the same page and to work together—I don’t want any loose cannons causing chaos.

Etymology

Origin of loose cannon

First recorded in 1975–80

Example Sentences

See Examples For:

loose cannon

During that time, he made a slew of enemies — from mob figures who accused him of fabricating evidence to fellow law enforcement officials who described him as a glory-seeking loose cannon.

From Los Angeles Times ● Aug. 13, 2026

“Currently the logic is not to escalate, and when you put in a player that is a loose cannon in the mix, you lose control of the situation.”

From The Wall Street Journal ● Jul. 21, 2026

His mother doesn't acknowledge that Ruben is a loose cannon loaded with vast quantities of gunpowder.

From BBC ● Apr. 24, 2026

Nixon and Kissinger saw him as a loose cannon, and they wondered whether he was divulging these secrets, possibly to foreign agents.

From Slate ● Jun. 16, 2023

“You and I never got along. Why would you want a loose cannon on your team?”

From "The Son of Neptune" by Rick Riordan