My granny is 90 and when she's behind the wheel she's a loose cannon

Mia nonna ha 90 anni e quando è al volante è un pericolo ambulante

You'd better stay at home. You are a loose cannon: with every step you take something happens

È meglio che tu te ne stia a casa, sei un pericolo ambulante per la comunità. Ogni passo che fai succede qualcosa

On the old-time warship the cannon was mounted on a wheeled carriage, which between battles was securely lashed to the ship's structure. If during a storm the lashings on a gun broke, it became a loose cannon: a ton of metal on wheels, rolling unpredictably around the deck