This phrase is used to describe someone who is unstable, unpredictable, or uncontrollable. This term has its roots in Naval warfare in the Age of Sail. Due to their weight the cannons on a sailing ship, which were generally referred to as guns, were secured to prevent them from moving about with the waves of the sea or to prevent damage from the enormous recoil when fired. A cannon that had become loose of its restraints and was rolling dangerously about the deck could cause immense damage to the vessel and crew.