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.