loose cannon

In the days of sail, naval vessels mounted cannon carefully and purposefully rigged into positions which optimised the effect of their fire. Occasionally, the rigging of these very heavy guns might come loose, and with the vessel pitching about on high seas or during manuever in battle, the results for the ship and crew could prove catastrophic. The term has come to describe a person lacking prudence or insight, whose actions and/or speech jeopardises the safety of people in their proximity or under their authority.