We all know someone like this. But many people think that the term loose cannon, meaning someone who is unpredictable, has something to do with men who carry guns. This is because the phrase is used so often in westerns and crime movies etc. The phrase actually has nautical roots. Cannons aboard a ship were very heavy and dangerous. They had to be properly secured or else they could either become thrown around the deck during rough weather or cause injury due to the massive recoil action during firing.