loose cannon

Someone who has the propensity to act unpredictably or to lose his or her 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 person whose actions are unpredictable and uncontrollable. As it turned out, he's not just a loose cannon. He makes sense. Some loose cannon in the State Department has been leaking stories to the press.

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.