Someone who behaves unpredictably and can cause problems.
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A person you can't predict, who may cause trouble at any moment.
A loose cannon acts alone and without thinking, and other people pay for it.
The phrase points to a lack of control.
"Wild Card"
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Someone unpredictable whose outcome could be positive or negative
"Hothead"
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Someone quick to anger, not necessarily unpredictable in all areas
"Loose Cannon"
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An unpredictable person whose actions are risky and hard to control