The definition of a loose cannon is someone or something unpredictable who could do something destructive or dangerous at any time.
An example of a loose cannon is your drunk uncle at your wedding who may or may not start to yell at everyone and throw a fit.
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a careless, uncontrollable person using poor judgment, whose unpredictable words or actions cause embarrassment or harm to others
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
One that is uncontrolled and therefore poses danger:
“ [His] bloopers in the White House seem to make him . . . a political loose cannon” (Tom Morgenthau). Origin: From the threat posed by loose cannon rolling about a warship under sail. The American Heritage® Dictionary of the English Language, 4th edition Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved. loose cannon - Phrases/Idioms
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® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.