- 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.
loose cannon definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
a careless, uncontrollable person using poor judgment, whose unpredictable words or actions cause embarrassment or harm to others
loose cannon definition by American Heritage Dictionary
noun SlangThe 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.
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.
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.