Idiom of the week
27. Loose cannon
"Hey George, see that bloke over there by the bar?"
"Isn't he that bloke who just came out of prison. The one on the telly."
"Charles Bronson. Yeah. Stop looking at him. He's a bit of a loose cannon. He'll be all nice one minute and break your legs the next."
"I'm going to go over and say hello."
"Are you crazy? He'll kill you!"
"I've never met a celebrity."
"Well, you're going to meet Jesus himself soon enough if you go over there."
"Here, hold my pint."
"Don't.! Oh my God. Jackie, better get on the phone. Call the ambulance."
Explanation: A person who is very unpredictable in a bad way. (S)he can be very violent or lose control very suddenly.
Notes: Cannons used to be used in sea battles, and, because they were very heavy guns, had to be very tightly tied up at all times. To have a cannon that wasn't tied up (a loose cannon) was very dangerous for everyone on the ship.