The definition of a loose cannon is someone or something unpredictable who could do something destructive or dangerous at any time.(noun)
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.
See loose cannon in Webster's New World College Dictionary
See loose cannon in American Heritage Dictionary 4
Origin: From the threat posed by loose cannon rolling about a warship under sail.
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