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

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

From BBC • Apr. 24, 2026

“He had to protect himself from his loose cannon partner over here,” he said.

From Salon • Jan. 21, 2026

That’s not to say that Harris is best as a loose cannon.

From Los Angeles Times • Sep. 13, 2024

“One man’s loose cannon is another man’s freedom fighter,” she wrote in 1987 of the city’s cast of characters in 1987.

From Slate • Jun. 14, 2023

“If you want to tie down a loose cannon, you can’t give it much rope, follow?”

From "Dry" by Neal Shusterman and Jarrod Shusterman