To be a loose cannon refers to a person who should be controlled otherwise that person may cause damage. It refers to the fact that if a cannon is not controlled properly, then the cannon ball may go off target and create a lot of damage.
1. Bharat was a very good soccer player but he was often a loose cannon on the field and needlessly got into arguments with opponent players and referees.
2. Shivam's father realized that if he did not discipline his son early on, he could become a loose cannon in later years.
3. The forest ranger decided to take the quick decision of shooting down the wild bear before it attacked the camp, as it had obviously become a loose cannon.