a person whose reckless behavior endangers the efforts or welfare of others.
Origin of loose cannon
First recorded in 1975–80
Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for loose cannon
a person or thing that appears to be beyond control and is potentially a source of unintentional damage
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Idioms and Phrases with 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.
The American Heritage® Idioms Dictionary
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