1. noun
a) a cannon that breaks loose during battle or a storm and causes serious damage to the ship and its crew
Jack is considered a loose cannon due to his volatile personality and track record of demonstrating a lack of ability to maintain composure.
b) an uncontrolled or unpredictable person who causes damage to his own faction, political party etc.
2. verb
To behave in a way such as to cause damage to ones own faction, political party etc.
Geralds loose cannoning triggered a volley of faithfully quoted media coverage which blasted a hole in his companys profits .
Wikipedia foundation.