1
general::
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.
American Heritage Idioms
2
general::
Phrase(s): a loose cannon
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.
McGrawhill's American Idioms And Phrasal Verbs