a 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.
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.
a loose cannona 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.
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.
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