- 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
- He was a loose cannon and could not be risked in front of the press.
- I don't think he's just a loose cannon, sometimes he really makes sense.
- The candidate turned out to be a loose cannon, and most of the voters could not place their trust on him.
- He was considered to be a loose cannon due to his volatile temper.
- They did not share any secrets with him since he was seen as being something of a loose cannon.
- He is a loose cannon and would not be a good choice for a leader.
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.
The phrase first appeared in French in Victor Hugo's novel "Ninety Three" in 1874. Henry Kingsley's novel "Number Seventeen" in 1875 refers to Hugo's phrase and is the first usage in English. Both these citations were in the literal form. The earliest figurative use is from The Galveston Daily News in December 1889.