a person whose reckless behavior endangers the efforts or welfare of others.
Origin
1975-80
British Dictionary definitions for loose cannon
loose cannon
noun
1.
a person or thing that appears to be beyond control and is potentially a source of unintentional damage
Slang definitions & phrases for loose cannon
loose cannon
noun phrase
A person who is quite likely to cause damage; a wildly irresponsible person: Haig is a loose cannon on a pitching deck/ His detractors call him a loose cannon who makes recommendations in public before consulting(1977+)
Idioms and Phrases with loose cannon
loose cannon
One who is uncontrolled and therefore a serious and unpredictable danger. For example, We can't trust her to talk to the press—she's a loose cannon. This metaphoric expression alludes to cannon mounted on the deck of a sailing ship, which if dislodged during combat or a storm could cause serious damage to both vessel and crew by sliding about. Its figurative use dates from the first half of the 1900s.