Idiom: Loose cannon Posted on 1st June 201831st May 2018 by DoubleClick Loose cannon: An unpredictable or uncontrolled person who can cause damage if not kept in check Example: As it turned out, John is not just a loose cannon, but he makes sense. Share this:Click to share on Twitter (Opens in new window)Click to share on Facebook (Opens in new window) Related Posted in Idiom and tagged Idiom L.