【Definition】Somebody whose activities are uncommon and out of control, an uncontrolled person who can cause unintentional damage. Someone who behaves in an unexpected and uncontrolled way and is liable to cause problems.【Origin】The phrase refers to cannons carried by wooden warships in the 17th to 19th century as their primary weapons. In order to avoid the enormous recoil when fired, these cannons were mounted on rollers and secured with ropes. A cannon that was not thus restrained was called a loose cannon and was considered dangerous.这个俗语反映了两百年前打仗的形式。loose 是松散的意思，而 cannon 是指大炮。Loose cannon 的来历是几百年前当人们在海上作战时，他们往往在船上放好些大炮，每一座大炮放在一个带四个轮子的平台上。在战争间隙的时候，船员们用很粗的绳子把这些大炮拴住，不让它们随便移动。可是，当风暴来临，这些绳子在风浪的冲击下断了以后，大炮就会在甲板上到处滚动，会把那些不幸就在大炮旁边的船员轧死，这比真正的敌人还要危险。现在，人们用 loose cannon 来指”一个失去控制、无视权威、打破常轨的人”，这种人往往会伤害和他自己站在一边的人。例如，有些美国政府官员在重大问题上擅自采取行动，也不向总统汇报。国会议员就把这些人称为 loose cannon。