A loose cannon
/ə luːs ˈkænən/informalUK · US
What does “A loose cannon” mean?
A loose cannon means an unpredictable person whose uncontrolled actions may cause problems for others.
A loose cannon is someone whose unpredictable actions may cause trouble for other people. They might make promises without asking or reveal something they should keep private.
It is a criticism, not simply another way to say “independent thinker.” The risk to others is part of the meaning.
When do people say it?
Use it critically about someone whose behaviour makes them difficult to trust with responsibility.
“He knows the subject, but he's a loose cannon who may promise things the team cannot deliver.”
Is it formal or informal?
Register: informal. It is most at home in everyday conversation rather than formal writing. Most common in UK and US.
Where does it come from?
Picture a heavy cannon rolling loose on a ship's deck. It could cause damage at any moment, much like a person whose actions are hard to predict.
Origin confidence: Disputed; treat with caution
How do you say “A loose cannon” in other languages?
Related idioms
- Rock the boat, Cause trouble or disrupt a stable situation.
- Get out of hand, To become uncontrolled or wild
Last updated 2026-09-16