loose cannon

loose cannon (plural loose cannons)

1. (nautical) A cannon that breaks loose from its moorings on a ship during battle or storm, which has the potential to cause serious damage to the ship and her crew.
2. (idiomatic, by extension) An uncontrolled or unpredictable person who causes damage to their own team, faction, political party, etc.

   Jack is considered a loose cannon due to his volatile personality and his track record of being unable to maintain his composure.

loose cannon (third-person singular simple present loose cannons, present participle loose cannoning, simple past and past participle loose cannoned)

1. (idiomatic) To behave in a way such as to cause damage to one's own faction, political party etc.
   • 2006, Gerry McCusker, Public Relations Disasters: Talespin--Inside Stories and Lessons Learnt, page 179,

     Gerald's loose cannoning triggered a volley of faithfully quoted media coverage which blasted a hole in his company's profits […]