take a long walk off a short plank

To take a long walk off a short plank is a way to tell someone to leave in a very impolite manner. By telling them to take a walk off a short plank, they are referring to pirate ships where the person would walk off a plank to die as they would either die on impact or drown in the water. This is a slightly rude or even cruel way to tell someone you want them to leave and stay gone permanently.