Origin of Phrases - L
Loose cannon
Meaning: A person who is out of control, unpredictable, who may do damage.
Example: The typical Engineer is too honest for his own good, they can be like a loose cannon around customers.
Origin: On sailing ships that had cannons, it was important that they be secured. Cannons are very heavy, and a loose cannon on a ship's deck in a rough sea could be thrown about in an unpredictable fashion, causing a lot of damage.
More than just needing to be lashed down during normal travel, cannons needed to be secured during use, or else the recoil would send the cannon on its way causing injury or damage.