loose cannon

noun

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

loose cannon

noun

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

Word History and Origins

Origin of loose cannon¹

First recorded in 1975–80

Idioms and Phrases

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.

Example Sentences

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

Jackie Malecek, 50, of Waterloo said she likes Trump for the reasons that many people don’t — how outspoken he is and “that he’s a little bit of a loose cannon.”

From Seattle Times