loose cannon

US /luːs ˈkæn.ən/
UK /luːs ˈkæn.ən/
"loose cannon" picture
1.

难以捉摸的人, 不可预测的人

a person who is unpredictable and can cause damage if not controlled

:
He's a real loose cannon; you never know what he's going to do next.
他真是个难以捉摸的人;你永远不知道他下一步会做什么。
The new manager is a bit of a loose cannon, making decisions without consulting anyone.
新经理有点像个难以捉摸的人,不咨询任何人就做决定。