A hubcap is a part of a wheel for a car.
In 1986, Edna Strickland Tannen yelled at Tiff Tannen, who was messing with a hubcap outside her apartment.
In 1931, Marty throws a hubcap at Kid Tannen and dislodges his gun, subsequently using it to shoot Doc's shackles and escape from the paddywagon, saving Doc's life.