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.

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.

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

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

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
See also:
  • loose cannon
  • loose cannon, a
  • fly into a temper
  • beware of
  • beware of (someone or something)
  • blow (one's) cool
  • blow cool
  • blow one’s cool
  • blow one's cool
  • blow your cool
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.
Tories decide on Boris Johnson as new PM; Readers give their views on new leader
"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."
Hillary Clinton calls Trump a 'loose cannon' in final pitch to voters
The broadcaster said anything can happen during the GAA season and described the controversial commentator as "a loose cannon worth having".
'Joe is a loose cannon ..but he's worth having' Sports host Lyster backs outspoken GAA pundit Brolly
Ken admitted: "Tyson's a loose cannon and I think the problem is he's got no-one in PR to guide him.
Tyson ready to rumble in Brum; THE MIDLANDS BOXING BEAT WITH MIKE LOCKLEY
Some people think he's a loose cannon, others think...
Liverpool, Barcelona, Messi: Luis Suarez has made things WORSE for all of them; Liverpool had a better record when Suarez was NOT in their side, so do Barcelona and Messi's stats have dropped since the bitey one moved to Spain. So is Suarez just a bad luck charm?
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".
WAR OF WORDS
"On the pitch he's a great player that every coach would want in his team, but off it he's a loose cannon.
Loose cannon Odemwingie must grow up, says coach Siasia
Blatter is a bit of a loose cannon. He's an embarrassment to FIFA.
Brady has real blast at Blatter
"He (Blatter) is a bit of a loose cannon," said Brady.
Brady has blast at Fifa chief Blatter
"He (Blatter) is a bit of a loose cannon, He's an embarrassment to FIFA.
LIAM BRADY has branded FIFA [...]; ShortCorner
He said: "He (Blatter) is a bit of a loose cannon. He is an embarrassment to Fifa."
Brady: Blatter an embarrassment; FOOTBALL SHORTS: WORLD CUP
That's tough enough to achieve without a loose cannon like Kadafi flapping his gums.
U.S. editorial excerpts -8-
"I'd hear 'Loose cannon.' I don't think speaking honestly on any of the issues should characterize you as a loose cannon."
Too pro-gay for the party? Democratic firebrand Paul Hackett dropped out of politics after party leaders pressured him to make way for a "safer" U.S. Senate candidate in Ohio. Did his vocal support for marriage equality doom his candidacy?
Historic parallels always pose difficulties, but Ahmadinejad is clearly a loose cannon and an embarrassment.
Today vs. 1935: hyperbole or prescience?
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."
Robertson warns Dover's anti-ID voters not to expect protection from God