a loose cannon

loose cannon

Someone who has the propensity to act unpredictably or to lose their temper very quickly. You really have to be mindful of what you say to Jake. He's a loose cannon, and the smallest things will send him into a fit of rage. The star quarterback's reputation as a loose cannon hurt his chances of being signed by a new team.
See also: cannon, loose
Farlex Dictionary of Idioms. © 2022 Farlex, Inc, all rights reserved.

a loose cannon

COMMON If you call someone a loose cannon, you mean that their behaviour is unpredictable and could cause problems. He was also getting a reputation for being a loose cannon; an accident waiting to happen. Thomson can be a loose cannon — he's not easy to control. Note: This expression refers to the cannons which used to be carried on the decks of warships. If one of the cannons was not properly fastened down, it could spin round and make a hole in the ship.
See also: cannon, loose
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012

a loose cannon

a unpredictable person or thing likely to cause unintentional damage.
A loose cannon was originally a cannon that had broken loose from its fastening or mounting, an accident especially dangerous on wooden ships of war.
See also: cannon, loose
Farlex Partner Idioms Dictionary © Farlex 2017

a ˌloose ˈcannon

(disapproving) a person who behaves in a way that you cannot predict, often with serious or damaging results: He has a reputation as a loose cannon whose comments sometimes upset Wall Street.On a ship, a cannon that was not properly tied down was a danger as it could roll around and hurt people or damage the ship.
See also: cannon, loose
Farlex Partner Idioms Dictionary © Farlex 2017

loose cannon, a

A grave and unpredictable hazard. This term comes from the days of sailing ships, when cannon—guns for firing heavy projectiles—were mounted on deck. If during combat or a storm a cannon came loose from its mounting and rolled about the deck, it could severely damage the hull, causing the ship to sink, as well as injure the crew. In the twentieth century the expression began to be used figuratively for a person who behaves unpredictably and is potentially damaging, as in “The president’s secretary was a loose cannon—she couldn’t keep these matters confidential.”
See also: loose
The Dictionary of Clichés by Christine Ammer Copyright © 2013 by Christine Ammer
See also:
References in periodicals archive ?
"With his past record he isn't fit to be PM." Tom Naysmyth, from Wishaw, said: "I think he can be a loose cannon but he has been voted in and has to be given an opportunity.
"Between an economy that works for everyone or only those at the top, between strong, steady leadership or a loose cannon who could put everything at risk."
The broadcaster said anything can happen during the GAA season and described the controversial commentator as "a loose cannon worth having".
Ken admitted: "Tyson's a loose cannon and I think the problem is he's got no-one in PR to guide him.
After the legendary Australian coach had criticised the turnout at the ongoing World League Final, Hockey India secretary general Narinder Batra called Charlesworth " a loose cannon" who needed " mental and psychological treatment".
"On the pitch he's a great player that every coach would want in his team, but off it he's a loose cannon.
Blatter is a bit of a loose cannon. He's an embarrassment to FIFA.
"He (Blatter) is a bit of a loose cannon," said Brady.
"He (Blatter) is a bit of a loose cannon, He's an embarrassment to FIFA.
He said: "He (Blatter) is a bit of a loose cannon. He is an embarrassment to Fifa."
That's tough enough to achieve without a loose cannon like Kadafi flapping his gums.
Historic parallels always pose difficulties, but Ahmadinejad is clearly a loose cannon and an embarrassment.
One of the new elected Dover board members, Judy McIlvaine, called Robertson's comments "unbelievable." Mcllvaine told the Philadelphia Inquirer, "He's such a loose cannon. Perhaps it's an honor that we have attracted the ire of Pat Robertson."