loose cannon
noun
: a dangerously uncontrollable person or thing
Example Sentences
a politician who is regarded as a loose cannon by her colleagues
Recent Examples on the Web Key to this feat is the Wanderer, a loose cannon of a character played by Caplan (who may be better known as lead singer of the folk band Ben Caplan & the Casual Smokers). 
Celia Wren, Washington Post, 12 Sep. 2022 Since then, O’Donnell seems to have involved herself in one kerfuffle after another as a kind of notorious loose cannon, on both television and social media. Rachel Syme, The New Yorker, 7 Sep. 2022 Shauna isn’t a loose cannon like Natalie (Juliette Lewis) or a creep like Misty (Christina Ricci), but Lynskey’s performance is unpredictable in its own subtler ways. Angie Han, The Hollywood Reporter, 21 June 2022 Francis has always been anticipated to be a bit of a loose cannon on the stand because of his intimate knowledge of events, axe to grind with the Navy and braggadocios air. San Diego Union-Tribune, 2 May 2022 Last but certainly not least, Twitter is about to become a debt-laden business belonging to the world’s richest loose cannon. Laura Forman, WSJ, 27 Apr. 2022 Mitchell was a loose cannon aimed at men with plenty to hide. Mark Peikert, Town & Country, 24 Apr. 2022 At the same time, Musk can be a loose cannon on these calls. Chris Isidore, CNN, 20 Apr. 2022 This is federal judge, John Adams, and he is known for being a bit of a loose cannon. Laura Johnston, cleveland, 10 Mar. 2022 See More
Celia Wren, Washington Post, 12 Sep. 2022 Since then, O’Donnell seems to have involved herself in one kerfuffle after another as a kind of notorious loose cannon, on both television and social media. Rachel Syme, The New Yorker, 7 Sep. 2022 Shauna isn’t a loose cannon like Natalie (Juliette Lewis) or a creep like Misty (Christina Ricci), but Lynskey’s performance is unpredictable in its own subtler ways. Angie Han, The Hollywood Reporter, 21 June 2022 Francis has always been anticipated to be a bit of a loose cannon on the stand because of his intimate knowledge of events, axe to grind with the Navy and braggadocios air. San Diego Union-Tribune, 2 May 2022 Last but certainly not least, Twitter is about to become a debt-laden business belonging to the world’s richest loose cannon. Laura Forman, WSJ, 27 Apr. 2022 Mitchell was a loose cannon aimed at men with plenty to hide. Mark Peikert, Town & Country, 24 Apr. 2022 At the same time, Musk can be a loose cannon on these calls. Chris Isidore, CNN, 20 Apr. 2022 This is federal judge, John Adams, and he is known for being a bit of a loose cannon. Laura Johnston, cleveland, 10 Mar. 2022 See More Word History
First Known Use
1889, in the meaning defined above