Loose cannon
American SlangDefinition
An unpredictable, uncontrollable person whose impulsive actions can cause trouble or damage.
Examples
"Don’t bring Trevor to the negotiation—he’s a loose cannon and could blow the deal with one outburst."
"The coach benched him because he’s a loose cannon on the field, constantly drawing penalties."
"Our PR team is on edge; that board member is a loose cannon who might tweet something reckless."