Loose Cannon

A real and genuinely terrifying shipboard hazard. Heavy cannons on wooden warships were lashed down with rope, and in rough seas a cannon that broke free could roll violently across the deck with enough force to crush crew members or smash through the hull itself. The figurative sense, an unpredictable person capable of real damage, was powerfully cemented by a dramatic early scene in Victor Hugo's 1874 novel Ninety-Three, in which a loose cannon rampages across a ship's deck during a storm.