loose cannon

American  

noun

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

---

loose cannon British  

noun

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

---

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.

---

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

Cunha has been linked with a big-money move over the past year with Europe's top clubs keen, but he may be viewed as a loose cannon because of his antics.

From BBC

Some of the grapevine talk has gotten back to her — how she’s a loose cannon or stirring the pot to cover up for complaints accusing her of misconduct.

From Los Angeles Times

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

From Los Angeles Times

The Trump campaign appears to understand this reality, but its candidate has slipped back into loose cannon mode.

From Salon

They have also tended to produce loose cannons throughout the ranks.

From Salon