1.NounA cannon that breaks loose from its moorings on a ship during battle or storm, with the potential to cause serious damage to the ship and her crew.
2.NounAn uncontrolled or unpredictable person, especially considered liable to cause unintentional or indiscriminate damage.
"Jack is considered a loose cannon due to his volatile personality and his track record of being unable to maintain his composure."
3.VerbTo behave in a way such as to cause unintentional or indiscriminate damage.